US forces have a less credible military foothold in Syria than the Russians do, and could be expelled from the country after the defeat of ISIS, a top US general has said.

Russia, unlike the US, operates in Syria at the invitation of the Syrian government, said General Raymond 'Tony' Thomas, the head of the Special Operations Command, at the Aspen Security Forum in Colorado on Friday.

'Here's the conundrum: We are operating in the sovereign country of Syria. The Russians, their stalwarts, their backstoppers have already uninvited the Turks from Syria. We're a bad day away from the Russians saying, "Why are you still in Syria, US?" Thomas said, according to Newsweek.

'If the Russians play that card, we could want to stay and have no ability to do it,' Thomas continued.

US forces have a less credible military foothold in Syria than the Russians do, a top US general has warned. Pictured: Iraqi soldiers pose with a captured ISIS flag in western Mosul on June 17

Unlike Russian forces, US troops are not present in Syria at the invitation of Syrian President Bashar al-Assad (pictured in March)

The US has several hundred special operators in Syria under Thomas' command, mostly Marines and Army Rangers.

Like the US, Russia is involved in the fight against ISIS, which holds dwindling territory in Syria.

General Raymond 'Tony' Thomas, the head of the Special Operations Command

But Russia has entered the fight at the invitation of Syrian President Bashar al-Assad, while the US has backed separatists fighting both ISIS and Assad.

Thomas also revealed that the US has halted a CIA program to equip and train certain rebel groups fighting the government, adding that the decision was not made to appease the Russians.

The US now exclusively backs the Syrian Democratic Forces, the new name for Kurdish militias in northern Syria.

Thomas said that the group 're-branded' at his urging, due to tensions with NATO ally Turkey, which has grappled with Kurdish separatist movements.

'I thought it was a stroke of brilliance to put democracy in there somewhere. But it gave them a little bit of credibility,' Thomas said.

Separately, Republican and Democratic lawmakers in Congress have reached agreement on legislation that allows new sanctions against Russia.

Congress has reached an agreement on legislation that allows new sanctions against Russia. Pictured: Russian President Vladimir Putin (right) with the Azerbaijani President on Friday

The bill would also give Congress veto power over easing existing sanctions, and could be sent to President Donald Trump for approval by the end of this month.

Many lawmakers hope the bill would send a message to Trump to keep a strong line against Russia.

Senate Democratic leader Chuck Schumer said a strong sanctions bill 'is essential'.

'I expect the House and Senate will act on this legislation promptly, on a broad bipartisan basis and send the bill to the President's desk,' Schumer said in a statement.

Senator Ben Cardin, ranking Democrat on the Senate Foreign Relations Committee, said the agreement was reached after 'intense negotiations.'

'A nearly united Congress is poised to send [Russian President Vladimir] Putin a clear message on behalf of the American people and our allies, and we need President Trump to help us deliver that message,' he said in a statement.