U.S. Secretary of State John Kerry is urging the Syrian opposition to join next week's peace talks in Geneva.
The Western-recognized Syrian National Coalition has said it would announce Friday whether or not to go to Geneva. Opposition leaders have expressed misgivings about sitting at a peace table with Syrian government officials.
Kerry is appealing to them not to miss an opportunity to form a transitional government. All sides must agree on the make-up of the new government, meaning Syrian President Bashar al-Assad would likely be excluded.
Kerry is rejecting Mr. Assad's attempts to shift the focus of the talks from forming a new government to fighting terrorism.
Mr. Assad considers the Syrian rebels terrorists. Kerry says the president is trying to undermine the peace talks.
Also Thursday, the U.N. human rights office said executions carried out by Syrian rebels, especially an al-Qaida-linked group, could amount to war crimes.
U.N. High Commissioner for Human Rights Navi Pillay said there were reports from the cities of Aleppo, Idlib and Raqqa of mass executions of civilians and fighters no longer part of the war.
The British-based Syrian Observatory for Human Rights said the two weeks of fighting among the various rebels forces in northern Syria has killed nearly 1,100 people, including 130 civilians.
Meanwhile, the United Nations delivered aid to two besieged areas under opposition control near Damascus Thursday. Syrian authorities promised this week to let humanitarian supplies into areas cut off by fighting.
A U.N. spokesman said a convoy assisted by the Syrian Arab Red Crescent had brought food, medicine and winter relief supplies to Al Ghuzlaniyah, close to Damascus airport.