The United States has potential military options against the Iranian-backed forces that are operating in Syria, either through direct strikes or by seeking to interdict supply flights that cross Iraqi airspace. But American officials made clear that they had no desire to use force against Russian troops in the country. Instead, the president could consider a range of other options to hold Moscow responsible, including further economic sanctions or diplomatic isolation.
Some lawmakers urged Mr. Trump to use sanctions authorized by Congress last year with bipartisan support. “The Syrian regime cannot exist without the support of Russia and Iran,” Senator Benjamin L. Cardin of Maryland, a senior Democrat on the Senate Foreign Relations Committee, said on CNN. “And we need to take action against both Russia and Iran, making it clear that they need to act to control what President Assad is doing.”
But some national security veterans doubted Mr. Trump’s tough words would amount to anything more than that. “Where we may come out with the Iranians and Russians is it’s mostly talk, and he doesn’t have a plan or they don’t have a plan because I just can’t imagine they want to escalate into a military situation,” said Robert S. Ford, a former ambassador to Syria.
Mr. Trump left little doubt that he intended to order a military response against Syria for what he called a “barbaric act” and said a decision could be made as early as Monday night or soon thereafter. “We’re making a decision as to what we do with respect to the horrible attack that was made near Damascus, and it will be met and it will be met forcefully,” he said Monday evening as he hosted the military leadership for dinner at the White House.
The White House was feeling pressure from France to act, lest President Emmanuel Macron do so first, according to a Trump administration official who spoke on the condition of anonymity to describe delicate international interactions. Mr. Macron, who spoke with Mr. Trump by telephone on Sunday and again on Monday, has repeatedly declared that the use of chemical weapons by Syria’s government would be a red line and pledged to strike weapons sites connected to such attacks.
Two Arleigh Burke-class guided-missile destroyers are in the Sixth Fleet’s area of operations in the Mediterranean Sea and would be able to get within striking range within hours to days. When Mr. Trump ordered the retaliatory strike against Syria at almost the exact same time last year, it was carried out by two destroyers firing 59 Tomahawk cruise missiles at Al Shayrat airfield, the suspected source of the chemical attack, hitting fighter jets, hardened aircraft shelters, radar equipment, ammunition bunkers and sites for storing fuel and defense systems.
The already tense situation in the Middle East was further inflamed early Monday morning by an attack reportedly conducted by Israel on a Syrian air base used by Iranian-backed militias. The strike killed about 14 people, according to a conflict monitoring group, and Russia and Syria said it was carried out by Israel, whose government declined to confirm its involvement.
The chemical attack in the suburb of Douma over the weekend killed at least 49 people and raised the temperature of an already simmering relationship between the United States and Russia, which rejected the conclusion that Syria’s military was behind the chemical attack. It asserted that the attack was staged by militants to falsely blame the government and justify an American strike against Mr. Assad’s government.
Russia’s foreign minister, Sergei V. Lavrov, told reporters in Moscow that despite Mr. Trump’s comments last week that he wanted to withdraw American troops from Syria, the United States was actually seeking to entrench itself in the country. “The U.S. is taking steps not to leave as President Trump said, and leave Syria for others, but to establish a foothold there for a very long time,” Mr. Lavrov said.
Mr. Trump dismissed the Russian and Syrian denials. “They’re saying they’re not” responsible, “but to me, there’s not much of a doubt,” he told reporters. Mr. Trump said that Syria was not allowing any independent inspection of the attack site. “If they’re innocent, why aren’t they allowing people to go in and prove” it, he asked.
France was not the only European ally to express outrage over the attack. “If they are found to be responsible, the regime and its backers — including Russia — must be held to account,” Prime Minister Theresa May of Britain said during a visit to Denmark.
At the United Nations, the United States called for an investigation into the site of the attack. Ambassador Nikki R. Haley used an emergency meeting of the Security Council to blame Russia and Iran for “enabling the Assad regime’s murderous destruction.”
“History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria,” she said. “Either way, the United States will respond.”
The United States floated a draft resolution on Monday to establish an independent international panel to look into who used chemicals weapons, which are prohibited by international law, but there is little chance that Russia would agree to it. Late last year, Russia effectively disbanded a previous panel that had been set up to identify perpetrators of chemical weapons strikes inside Syria.
The panel, known as the Joint Investigative Mechanism, looked at eight cases of alleged chemical weapons use in 2015 and 2016, concluding that the Syrian government had used chemical weapons in three attacks and that the Islamic State had used them once. The panel’s mandate expired, and Russia blocked an effort to extend it, calling it a tool of Western powers.
Vassily A. Nebenzia, the Russian ambassador to the United Nations, insisted on Monday that no chemical weapons attack took place at all in Douma, arguing that it had been fabricated to build an “anti-Russian alliance.” He said the Organization for the Prohibition of Chemical Weapons could visit Damascus, with Russian help, and carry out its investigation.
Mr. Nebenzia went on to boast of Russia’s popularity. “You’re misguided if you think you have friends,” he told Ms. Haley, adding, “Russia has friends. Unlike yourselves, we do not have adversaries.”
In Geneva, Zeid Ra’ad al-Hussein, the United Nations high commissioner for human rights, issued a scathing denunciation of the feeble response to past attacks by the international community in general.
Warring parties in Syria had used chemical weapons at least 35 times since 2013, Mr. al-Hussein said, summing up the response of the international community as “empty words, feeble condemnations and a Security Council paralyzed by the use of the veto.”
“The world — and in particular the veto-wielding states on the Security Council — need to wake up, and wake up fast, to the irreparable damage that is being done to one of the most important planks of global arms control and prevention of human suffering,” he warned.
The crisis came on a day when Mr. Trump’s foreign policy team was in the midst of transition as his new national security adviser, John R. Bolton, arrived for his first day on the job. Still adjusting to the assignment, Mr. Bolton sat back as Vice President Mike Pence led a national security meeting, but he sat next to Mr. Trump for his dinner with the generals. Mr. Trump was expected to huddle with Mr. Bolton, Defense Secretary Jim Mattis and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, after dinner.
Known as a national security hawk, Mr. Bolton in the past urged military action against governments in Iran and North Korea to counter their nuclear programs, and he remains a staunch defender of the 2003 invasion of Iraq to topple Saddam Hussein.
But like Mr. Trump, he resisted a strike against Syria when President Barack Obama was in office and facing a similar choice after a chemical weapons attack against civilians in 2013. In that instance, Mr. Obama sought support from Congress, but ultimately backed off a strike after reaching an agreement with Russia to remove Syria’s chemical weapons arsenal.
“If I were a member of Congress, I would vote against an authorization to use force here,” Mr. Bolton said at the time. “I don’t think it’s in America’s interest. I don’t think we should in effect take sides in the Syrian conflict.”
Mr. Bolton took a different position when Mr. Trump ordered action last year. “I think the Trump decision to strike as they did was the correct decision,” he said then. He added, “I think there is an American national interest in preventing people from violating treaties that try to restrict the use or the spread of weapons of mass destruction.”