European Union nations have criticised Russia's military intervention in Syria, with the bloc's top diplomat calling it a worrying "game changer".
But EU countries maintained Moscow's efforts would not keep president Bashar Assad in power.
EU foreign ministers warned Moscow to centre its military actions in Syria on Islamic State and not go after the moderate opposition at the same time.
"It has to be co-ordinated" among the US, the EU and Russia, EU foreign policy chief Federica Mogherini said of the attacks in Syria.
"Otherwise it risks being extremely dangerous, not only from a political point of view, but mainly from a military point of view."
Russian air strikes have backed moves by Syrian troops against insurgents in the centre of the country, but President Vladimir Putin has said Russian efforts would help reach a political settlement.
Countering this, the EU ministers said in a statement that "this military escalation risks prolonging the conflict, undermining a political process, aggravating the humanitarian situation and increasing radicalisation".
The EU said that Assad could not be part of a solution to the conflict - at most an inevitable partner in the transition from war.
"There cannot be a lasting peace in Syria under the present leadership," according to the conclusions of the EU meeting.
"There will be no peace in Syria if Bashar Assad, who is responsible for most victims of this civil war, can stay," French European affairs minister Harlem Desir said.
Even if France does not want Assad represented in Syria's moves towards peace, others see his position as inevitable, especially with Russian backing.
"It's not about who we like and who we don't like," Slovakian foreign minister Miroslav Lajcak said. "It's about who is relevant and who is not. And as of today, he definitely is relevant," he said of Assad.
And German foreign minister Frank-Walter Steinmeier acknowledged that "the position and importance of individual actors has changed. With Russia's involvement, much has changed".
"There is no question that the efforts to arrive at a political solution have become more difficult and more complex," he said.
Ms Mogherini said that "it is for sure a game changer. It has some very worrying elements".
Meanwhile, Nato secretary general Jens Stoltenberg accused Russia of prolonging the war in Syria by supporting Assad and called on Moscow to join the fight against IS militants.
Mr Stoltenberg said Russia's violation of Turkish and Nato airspace is unacceptable and that the alliance is worried about Russia's "substantial military build-up, its air strikes and its cruise missile attacks" in the Middle East.
Noting that many countries in the region and Nato allies are participating in the US-led mission against the militants, he called on Moscow to "play a constructive role in the fight against Isil".
He added that "supporting the Assad regime is not constructive .... (and) is only prolonging the war".
Mr Stoltenberg spoke at Nato's parliamentary assembly in the south-western Norwegian port of Stavanger.