Pompeo came under fire at a Senate hearing over President Donald Trump's statement that he had not raised with Putin accusations that Moscow paid the Taliban bounties to kill US troops in Afghanistan.

"I don't think there's any doubt in the mind of every Russian leader, including Vladimir Putin, about the expectations of the United States of America not to kill Americans," Pompeo told the Senate Foreign Relations Committee.

"I can promise you that the 300 Russians who were in Syria and who took action that threatened America who are no longer on this planet understand that, too," he said.

Pompeo did not specify an incident but there have been multiple reports that US airstrikes killed Russians in February 2018 near the Syrian town of Khasham.

The New York Times, which first reported the alleged Russian bounties in Afghanistan, said Moscow may have been retaliating for the strike in which hundreds of pro-Damascus fighters were believed to have been killed, including numerous Russian mercenaries.

Russia intervened in Syria in 2015 to defend President Bashar al-Assad and had largely avoided direct clashes with the United States, which sent troops and launched airstrikes aimed at destroying the Islamic State group.

Trump, in an interview this week with Axios, said that US intelligence did not think the account of Russian bounties in Afghanistan was real and that he never raised the issue with Putin.

Senator Robert Menendez, the top Democrat on the committee, told Pompeo that Trump's remarks were "astonishing" and that the bounties marked an "outrageous escalation" by Russia.

Menendez asked Pompeo, a former Republican congressman known for his blistering attacks on Democrats, how he would have acted if a Democratic president acted similarly to Trump.

"If you were sitting in your old House seat, would you be okay with a president who abandoned our troops but not even raising this with the Kremlin?" Menendez said.

Pompeo said he had raised Afghanistan with Foreign Minister Sergei Lavrov but declined to discuss Trump's discussions.