Secretary of State Rex W. Tillerson characterized the strike as an “overwhelming success” and said Americans should be proud of the “overpowering” force of the American military. Treasury Secretary Steven Mnuchin announced that the United States would impose additional sanctions on Syria, but he did not discuss the timing or targets.

But the strike inserted the United States, for a moment at least, into one of the world’s most intractable conflicts and demonstrated the potential dangers of Russian and American forces’ operating in proximity. As many as 100 Russian troops were believed to be stationed at the Syrian air base targeted on Thursday. An American official said the Russians on the ground had been given 60 to 90 minutes’ notice that the missiles were coming and had not been advised whether to take shelter or flee.

Although Russia did not deploy its air defense system in Syria against the American missiles, it flexed its military muscles after the attack. Moscow said it would bolster Syria’s air defenses, and the Russian news agency Tass reported that a frigate would enter the Mediterranean Sea on Friday and visit the logistics base at Tartus, a Syrian port.

The Russian military said it would shut down a hotline established to prevent accidental clashes in the skies over Syria. While the two sides used the channel earlier on Friday, Russian officials said it would be cut off at the end of the day. The United States and Russia have other ways to track each other’s aircraft and avoid collisions, but American officials considered the hotline an important vehicle to ensure safety, as well as a valuable political connection.

Even as Moscow protested, American officials pointed fingers back, faulting the Kremlin for not enforcing a 2013 agreement it brokered with Syria to eliminate its chemical weapons. “Clearly, Russia has failed in its responsibility to deliver on that commitment from 2013,” Mr. Tillerson said late Thursday night. “So either Russia has been complicit or Russia has been simply incompetent in its ability to deliver on its end of that agreement.”

Russia, which sent armed forces to Syria in 2015 to bolster Mr. Assad against insurgents, denied that his government was behind the chemical attack on Tuesday in Idlib Province that left more than 80 people dead, calling that a pretext. Instead, Moscow said a conventional strike had hit a chemical weapons warehouse controlled by insurgents, an explanation dismissed in the West.