WASHINGTON: Some 40,000 Syrian troops have quit their government's crackdown on rebels against President Bashar Assad's rule, Republican US Senator John McCain said Thursday.

McCain said he was surprised by the figure, which visiting Turkish Foreign Minister Ahmet Davutoglu provided when the two officials met in the US Congress amid fears that the bloodbath in Syria shows no sign of slackening.

"He did say that some 40,000 Syrian military have defected. I didn't know that number was that large," the lawmaker, his party's top member on the Senate Armed Services Committee, told reporters.

"Some of them have gone home, some of them have joined" rebel forces, said McCain, who underlined that nonetheless "the situation is worsening" as Assad's forces aim to crush a year-old uprising.

The lawmaker underline that Turkey, a US NATO ally, had been in communication with Russia, which Washington has denounced over Moscow's veto of a UN resolution aimed at defusing the crisis.

"They've been in communication with the Russians and expressed, obviously, their desire that Russia play a more constructive role," but Davutoglu "did not say" whether those efforts were bearing fruit, said McCain.