MOSCOW — Russia on Friday froze a critical agreement on military cooperation with the United States in Syria after an American military strike, warning that the operation would further corrode already dismal relations between Moscow and Washington.

Syria, Russia’s ally, condemned the American strikes as “a disgraceful act.”

In addition to suspending the pact to coordinate air operations over Syria, an accord that was meant to prevent accidental encounters between the two militaries, Russia also said it would bolster Syria’s air defense systems, and was reported to be planning to send a frigate into the Mediterranean Sea to visit the logistics base at the Syrian port of Tartus.

Dmitri S. Peskov, a spokesman for President Vladimir V. Putin of Russia, said that the point of the agreement had been to decrease danger in the air and that canceling it would not significantly increase that peril with missiles already flying around.

“Amid the missile strikes, it is hardly reasonable to talk about any more increase in the risk, as the risk has increased considerably,” Mr. Peskov said at a news briefing.