The US fears that while Russia says it plans to strike "Islamic State" (IS) targets in Syria, Moscow could direct its warplanes and other military might towards Syrian rebels, Carter told reporters on Thursday.

The move would weaken the rebels' chances of removing President Bashar Assad's government, he added.

His comments follow Russia's increased military presence at a Syrian coastal air base and President Vladimir Putin's vow to tackle IS, which controls large areas of Syria and Iraq.

Pentagon officials say it remains unclear how the Russians intend to use their military in Syria, which involves the deployment of fighter jets, tanks, helicopters, air defense missiles and other equipment.

Moscow remains a long-time ally of Assad's regime despite enormous international pressure for the Alawite leader to step down.

Fanning the flames

"To pursue the defeat of IS without at the same time pursuing a political transition is to fuel the very kind of extremism that underlies IS," Carter said, adding that Russia's strategy was a "logical contradiction."

He said if Moscow was to actively intervene now, it would be "pouring gasoline on the civil war in Syria."

Washington has ruled out any cooperation with Moscow without an agreement to discuss removing Assad from power.

Russia has begun flying drone surveillance flights over Syria, ahead of what some US officials expect will be the launch of fighter aircraft in the coming days.

Meanwhile, Putin is due to meet US President Barack Obama on Monday on the sidelines of the United Nations General Assembly, to discuss Russia's involvement in Syria as well as the Ukraine crisis.

Involve Assad

In a taped interview with US television, Putin warned that any effort to destroy Assad's government would create a similar situation to other Middle East countries, "where all the state institutions are disintegrated."

He renewed his call for Washington to include Assad in the fight back against IS militants.

The White House said the US would not back a UN Security Council statement on countering terrorism proposed by Russia. The draft urges countries to fight extremist groups "in coordination with the governments of affected states," a reference to Assad's regime.

But German Chancellor Angela Merkel said she was open to negotiations with the Syrian government. Speaking after an EU migrant summit, she said "we have to talk to many stakeholders, including Assad."

German Foreign Minister Frank-Walter Steinmeier later said that talks with the Syrian regime "was not the question that should be brought first to the table."

mm/lw (AP, Reuters)