Damascus has agreed "in principle" to attend a conference proposed by Russia and the United States on ending Syria's civil conflict, Russia's foreign ministry said on Friday.

It was the first confirmation that President Bashar al-Assad's government would be willing to take part in the talks with the opposition.

But despite the announcement from Moscow, Damascus has not offered any definitive statement on the proposed talks.

Russia and the US joined efforts earlier this month to convene an international conference to bring representatives of the government and the opposition to the negotiating table. The aim of the talks would be to establish the outlines of a transitional government as a way out of the crisis.

More than 70,000 people have been killed and several million displaced since March 2011.

The US-Russian plan, similar to the one set out last year in Geneva, calls for talks on a transitional government and an open-ended cease-fire.

The Moscow announcement came after days of talks there between Syria's Deputy Foreign Minister Faysal Mekdad and Russian officials.

Earlier on Friday, Russian foreign ministry spokesman Alexander Lukashevich said Moscow had "received an agreement in principle from Damascus to attend the international conference".

"We note with satisfaction that we have received an agreement in principle from the Syrian government in Damascus to participate in the international conference, in the interest of Syrians themselves, to find a political solution," Lukashevich said.

He added, however, that it is impossible to set the date for the conference at this point because there is "no clarity about who will speak on behalf of the opposition and what powers they will have".

Lukashevich also said Moscow "was not encouraged" by the results of recent meetings of members of the Syrian National Coalition, the country's main opposition group that has called on Assad to step down.

In Turkey, where the Syrian National Coalition is holding a three-day conference, an opposition figure expressed doubts over Moscow's announcement, questioning why Damascus has said nothing on the subject.

"We would like to hear that statement from the Assad government. This has been related to us through the Russians, I have no idea why the Syrians are silent," said Louay Safi, spokesman for the National Coalition.

"We need more clarity. The announcement is still very vague. And why is it not made by Damascus?"

The US, along with key European and Arab supporters of Syria's opposition, said on Wednesday that Assad must relinquish power at the start of a transition period. Russia, however, has not committed to Assad's departure and the Syrian leader has said he will not step down before his term ends next year.

US Secretary of State John Kerry on Thursday acknowledged the difficulties of launching peace talks. "Nobody has any illusions about how difficult, complicated, what a steep climb that is," he said during a visit to Israel.

Fighting continued across Syria on Friday, and state media reported that rebels fired mortar shells at the central prison in the embattled northern city of Aleppo, killing and wounding several inmates.

The pro-opposition Aleppo Media Center said clashes were underway between rebels and government troops at the prison and that a large fire had broken out at the facility.

AP-Reuters-AFP