The Pentagon on Monday voiced skepticism about Russian President Vladimir Putin's announcement that he had ordered the partial withdrawal of troops from Syria.

Putin visited the war-torn nation Monday and said a "significant part" of the Russian troop contingent in Syria is heading home after their mission had been largely completed.But Pentagon spokesman Major Adrian Rankine-Galloway said such declarations were not necessarily reflected by action.

"Russian comments about removal of their forces do not often correspond with actual troop reductions, and do not affect US priorities in Syria," he said.

A US official told AFP that Putin was likely to carry out a "token withdrawal" of some aircraft, then follow up by demanding the United States pulls its forces out of Syria.

The US military last week said it would stay in Syria, where it is fighting the Islamic State group, as long as necessary to ensure the jihadists don't return.

The "coalition will continue to operate in Syria in support of local forces on the ground to complete the military defeat of ISIS and stabilize liberated territory, in turn allowing for displaced Syrians and refugees to return," Rankine-Galloway said.

Syria's conflict began in March 2011 with anti-government protests, but quickly spiraled into a bitter and complex civil war, with IS just one element.

The open-ended US commitment in Syria is likely to rile Russia, which since late 2015 has conducted a separate military campaign to prop up the regime of President Bashar al-Assad.

The size of the Russian deployment in Syria is not known, but independent Russian military expert Pavel Felgenhauer has told AFP that up to 10,000 troops and private contractors could have taken part in the conflict.

The Pentagon later announced that the two principal US-backed forces that have been fighting the Islamic State group in Iraq and Syria met at the border between the two countries.

When it first began its push for a "caliphate", IS had almost unimpeded flow across the desert border, but Iraq is now strengthening its border posts to make sure remaining IS fighters can't move between countries.

The Syrian Democratic Forces and Iraqi Security Forces met at the border "in order to reaffirm their commitment to ensure that ISIS does not take root again," said another Pentagon spokesman, Colonel Rob Manning.