BEIRUT, Lebanon — High-level diplomatic efforts to stop the fighting in Syria yielded mixed results on Saturday as President Bashar al-Assad shut the door on any immediate negotiations with the opposition and escalated a new military assault on the city of Idlib.

Mr. Assad told the United Nations envoy Kofi Annan that such talks would be fruitless as long as “terrorist groups” were operating in the country.

“No political dialogue or political activity can succeed while there are armed terrorist groups operating and spreading chaos and instability,” the state news agency, SANA, quoted Mr. Assad as saying.

Mr. Annan, a former United Nations secretary general, was sent to Damascus by the United Nations and the Arab League to try to negotiate a cease-fire. In a statement, he described the talks as “candid and comprehensive,” and said he had also met with opposition leaders and young activists. He was due to meet with Mr. Assad again on Sunday.