RIYADH, Saudi Arabia — An array of Syrian opposition groups agreed here on Thursday to form a new and more inclusive body to guide the diverse and divided opponents of President Bashar al-Assad in a new round of planned talks aimed at ending the Syrian civil war.

The formation of such a body has been seen by the United States and the opposition’s other international supporters as a prerequisite for new talks, and the new body appeared to fit the bill by pulling together political dissidents who have long distrusted one another as well as rebel groups fighting the Syrian Army.

“This is the widest participation for the opposition, inside and outside of Syria, and we have the participation of the armed groups,” said Hadi al-Bahra, a member of the exiled Syrian National Coalition who attended the two-day conference that produced the new body.

The agreement in Riyadh, which Secretary of State John Kerry called “an important step forward,” followed a truce between rebels and government forces in part of the strategic city of Homs, which a senior United Nations official said could serve as a building block for a broader cease-fire agreement, so long as the government can hold up its end of the deal as proof that it “cares about its people.”