The actual date of the conference has not been made clear amid few signs of any end to a conflict that has killed more than 90,000 people in 26 months.
Russian foreign ministry spokesman Alexander Lukashevich said Moscow had received the Damascus government's assurance it would take part in the talks during a visit this week by Syria's Deputy Foreign Minister Faisal Muqdad.
"We note with satisfaction that we have received an agreement in principle from Damascus to attend the international conference in the interest of the Syrians themselves finding a political path to resolve the conflict," Lukashevich told reporters.
Russia is seen as the nation exercising the most influence over the Syrian regime thanks to years of military and other cooperation that stretch back to Soviet times.
Moscow has in recent weeks also been keen to show its diplomatic muscle in the region by ensuring that Assad's representatives attend the conference after a failed effort at getting the conflict resolved in Geneva last June.
The announcement came as Syria's main opposition group held a second day of talks in Istanbul aimed at finding a joint approach to what has already been dubbed the "Geneva 2" talks.
The Western-backed National Coalition -- the main opposition group -- said it wanted to see "goodwill gestures" from Assad.
"We want to make sure that when we enter those negotiations the bloodshed in Syria will stop," Khaled al-Saleh, a spokesman for the group, told reporters in the Turkish city.
Assad's place in any new transitional team proposed by Moscow and Washington remains the main stumbling block to the talks taking place in the coming weeks.
Some rebel leaders have said that no negotiations were possible with Assad remaining in power. The Damascus government counters that it wants assurances that the president's role will not be a subject of debate in Geneva.
Divisions within the opposition's ranks about whether to attend the talks at which Assad would be represented translated Friday into scepticism about the regime's own commitment to the negotiating process.
"We would like to hear that statement from the Assad government," Coalition spokesman Louay Safi said in Istanbul.
"This has been related to us through the Russians, I have no idea why the Syrians are silent."
Some European media have reported that the meeting has been already tentatively scheduled for June 10 despite all the disagreements.
But Lukashevich said reports of a specific date for the conference "cannot be taken seriously" because the ranks of Assad's foes remained so split.
"We are again hearing about the precondition that Bashar al-Assad leaves power, and that a government be formed under the auspices of the United Nations," Lukashevich said.
And he condemned attempts by Western and regional powers to convene a human rights conference at the United Nations as a one-sided attempt to further discredit Assad.
The calls for the UN Human Rights Council to initiate a debate on the deteriorating situation in Syria were renewed on Friday by the ambassadors to the body from the United States as well as Turkey and Qatar.
Meanwhile -- in a dramatic and rare intervention -- Lebanon's President Michel Sleiman cautioned the Shiite group Hezbollah over its militia fighting alongside regime troops in Syria.
Hezbollah "should not get bogged down in the sands of dissension, whether in Syria or Lebanon," Sleiman said in a statement.
At least 23 people have been killed since Sunday in inter-confessional clashes in the Lebanese port city of Tripoli.
The United States on Friday said it was "deeply concerned" about the spillover violence, saying the clashes were "a stark reminder that the conflict in Syria poses an increasingly dangerous threat to Lebanon's stability and security."
Inside Syria, the Syrian army, backed by Hezbollah fighters, said it had captured much of the rebel stronghold of Qusayr in central Homs province in a massive assault.