BEIRUT, Lebanon — The head of Syria’s main exile opposition group added new conditions on Wednesday to the start of any negotiations aimed at ending the country’s civil war, further decreasing prospects that an internationally backed peace plan would progress anytime soon.

The new conditions, made by the president of the opposition Syrian National Coalition, Ahmad al-Jarba, in an interview with the Qatar News Agency, reflected a significant hardening of his position. He said that the opposition would not negotiate with President Bashar al-Assad or “his clique” and that talks could begin only when the military situation in Syria was positive for rebel forces.

In an interview with The New York Times on Saturday, Mr. Jarba said he was prepared to attend a proposed peace conference in Geneva with representatives of Mr. Assad and suggested that the government make “positive steps” to show good faith beforehand, like releasing prisoners.

In the Qatar News Agency interview, however, Mr. Jarba rejected the participation of Mr. Assad and members of his government.