The Russian military said on Monday that two Israeli F-15 war planes had carried out strikes on a Syrian air base on Sunday, the Interfax news agency reported.

Interfax cited the Russian Defence Ministry as saying the Israeli war planes had carried out the strikes from Lebanese air space, and that Syrian air defence systems had shot down five of eight missiles fired.

Asked about the Russian statement, an Israeli military spokesman said he had no immediate comment.

Syrian state TV said on Monday the United States was suspected of striking an air base hours after U.S. President Donald Trump warned of a "big price to pay" as aid groups said dozens of people were killed by poison gas in a rebel-held town.

Earlier, Syrian state TV said on Monday the United States was suspected of striking an air base hours after US President Donald Trump warned of a "big price to pay" as aid groups said dozens of people were killed by poison gas in a rebel-held town.

The United States denied attacking the Syrian base.

"At this time, the Department of Defense is not conducting air strikes in Syria," the Pentagon said in a statement.

"However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."

Syrian state TV said there were casualties in what it said was a suspected US missile attack on the T-4 airfield near Homs, which is close to the ancient city of Palmyra in central Syria.

"An aggression was perpetrated on T-4 air base in several strikes that is most likely to be an American attack," state television said in a news flash.