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.