Syrian President Bashar al-Assad, his opposition, U.S.-backed Kurdish forces and the Islamic State militant group (ISIS) all recognized Eid al-Adha.
Once a primary recipient of CIA assistance, the Free Syrian Army has since turned on U.S. efforts to battle ISIS using Kurdish militias.
The Syrian military once again faced off in Damascus with jihadist rebels attempting to strengthen the opposition's stance in international peace talks.
Russia, Iran and Turkey said last week they were ready to help broker a peace deal after holding talks in Moscow.
The Syrian army have made rapid gains against insurgents in the past two weeks and look closer than ever to restoring full control over Aleppo.
If the rebels take Jarablus, it would preclude an assault by Kurdish forces who hope to expand their reach.
Amnesty International documented 24 cases of abduction in Aleppo and Idlib governorates.
An opposition representative said the delegation might turn up if their demands were swiftly met, but the chances appeared slim.
The rebels want all sides in Syria's civil war to allow humanitarian access to those in need and cease attacks on civilians.
Jaysh al Islam, a Syrian insurgent group, welcomes Saudi Arabia’s cutting ties with Iran due to Tehran backing of Shiite militias.
Former Defense Secretary Chuck Hagel lets slip how Syrian policy was made on the hoof.
Al-Shadadi is a key logistics hub for the group.
It is hoped that the factions can form a united opposition ahead of January peace talks with the government.
Kerry also said the war in Syria could be weeks away from a major breakthrough.
The Free Syrian Army rebel group confirmed the death of its chief of staff, Basil Zamo, formerly a captain in the Syrian military.
Gains by ISIS north of Aleppo would threaten the supply lines of rival rebels inside the city.
The offensive, which both officials said would begin soon, would expand on a ground attack by the same alliance.