U.S. Defense Secretary Ashton Carter warned Thursday that Russia’s expanded military presence in Syria could worsen the conflict and empower the Islamic State group.

Speaking at a Pentagon briefing, Carter said that Moscow’s buildup in Syria could “pour gasoline on the (ISIS) phenomenon,” because of their backing of one of the extremist group’s top enemies -- the embattled government of Syrian President Bashar Assad.

Carter’s statements came after U.S. officials announced that President Barack Obama’s administration was pushing for a deal with Russia on both Syria and Ukraine, and that the topics would be the focus of talks next week between Obama and his Russian counterpart Vladimir Putin.

“To pursue the defeat of ISIL without at the same time pursuing a political transition is to fuel the very kind of extremism that underlies ISIL, and if that’s the Russian view that’s a logical contradiction,” Carter said. “And the way out of that contradiction is to pursue both of those in parallel. And on that basis I think we’re prepared to discuss a way ahead with Russia where the political and the military move in parallel.”

The Russian president confirmed that he intends to keep Assad in power. “There is no other solution to the Syrian crisis than strengthening the effective government structures and rendering them help in fighting terrorism. But at the same time, urging them to engage in positive dialogue with the rational opposition and conduct reform,” Putin told Charlie Rose in a CBS interview published Thursday. “It's only the Syrian people who are entitled to decide who should govern their country and how.”

Meanwhile, Carter maintained that the U.S. would not soften its stance against Russia’s actions in Ukraine, for its annexation of the Crimean peninsula and for its alleged support to separatist groups in the country’s east. The "ongoing discussions on Syria will not in any way take away from our strong condemnation of Russia's action in Ukraine, or change our sanctions and security support in response to those destabilizing actions," Carter said.

"The bottom line is this -- Russia must end its aggression in Ukraine, end its occupation and annexation of Crimea and uphold its commitments under the Minsk agreements," he added, referring to the ceasefire brokered last year.

U.S officials said that Russia has begun conducting drone surveillance flights over Syria, conducting reconnaissance ahead of expected fighter aircraft launches in the coming days. They added that there have so far been no problems or conflicts with the U.S.-led coalition conducting airstrikes against ISIS.

Meanwhile, State Department spokesman John Kirby also said in a separate press meet that Washington was concerned over reports that Russia was supplying fighter aircraft and surface-to-air missiles to Damascus, which lacks its own air force.

“I think there are legitimate questions that we continue to have about the kinds of capabilities we see flowing in there,” Kirby said. “Secretary Kerry will continue to have the conversations he needs to have on the diplomatic side to try to get better clarity and better understanding.”

Carter had previously said that he wished to see official U.S. support for Kiev in the form of lethal weaponry, a position that the current administration disagrees on.