A Syrian opposition leader is undermining chances for proposed peace talks by saying that foes of President Bashar Assad will only attend if they make military headway first, Foreign Minister Sergei Lavrov said Wednesday.

Ahmad Jarba, the new president of the opposition Syrian National Coalition, said Sunday that the coalition would not go to the conference that Russia and the United States were trying to convene in Geneva unless its battlefield fortunes improved.

"If you follow this logic, we will never manage to convene any conference," Lavrov said when asked about the issue at a news conference after meeting the foreign minister of Belarus.

Russia and the United States said on May 7 that they would try to bring Syrian government and opposition representatives together at a conference to try to end a conflict that has killed more than 100,000 people since March 2011, but no date has been set.

"Our Western partners took responsibility for getting the opposition to attend the Geneva conference without preconditions," Lavrov said. Russia says Assad's exit must not be a precondition for a peace process.

Russia, which has given Assad crucial diplomatic support, has suggested that the main obstacles to the conference are disunity and a lack of commitment among the opposition.

Diplomats say recent successes by Assad's forces have made him less likely to seek compromise off the battlefield, while also making his opponents reluctant to negotiate without first gaining a stronger position.

Lavrov said an impasse in the U.S. Congress over White House plans to send arms to Syrian rebels would have little effect on the opposition's position on the proposed peace conference.

"They are getting enough weapons of various kinds … and they are using these weapons actively," he said.

Also on Wednesday, the opposition Syrian National Coalition denied Russian allegations that rebel fighters fired a projectile laden with the nerve agent sarin at a suburb of Aleppo in March, saying UN inspectors should be allowed to investigate the attack.

Separately, a Western diplomat said senior UN officials would head to Damascus soon to discuss ways of breaking the deadlock on access for a UN chemical weapons investigation team that has so far been unable to visit Syria.

Russia Says Syrian Opposition Blocking Peace Conference