BEIRUT >> The Iranian Foreign Ministry on Saturday welcomed a U.S.-Russian agreement on a cease-fire for Syria, where it has been a key ally of President Bashar Assad during the five-year war that has resulted in as many as 500,000 deaths.

Iran’s semi-official ISNA news agency quoted Foreign Ministry spokesman Bahram Ghassemi as saying that “Iran has always welcomed a cease-fire in Syria and the facilitation of humanitarian access to all people in this country.”

The agreement is set to go into effect on Monday night, coinciding with the Muslim holiday Eid al-Adha. Both Assad’s forces and rebels would halt attacks, while the U.S. and Russia would join forces against the Islamic State group and al-Qaida-linked militants in Syria.

With Iran’s endorsement, the agreement is now backed by Assad and all key allies of the Syrian government: Moscow, Tehran, and the Lebanese militant group Hezbollah. Iran has sent some of its top military commanders, as well as Revolutionary Guard and regular army forces, to bolster Assad’s troops.

Rebel factions say they are still deliberating the agreement, which permits government forces to strike al-Qaida-linked militants for seven days before the U.S. and Russia would take over the task.

Rebels have been fighting side-by-side with the al-Qaida-linked Jaysh Fatah al-Sham around the northern city of Aleppo as they try to break a government siege on the city’s opposition-held quarters. It is not clear how the government will distinguish between the two, and whether it can strike at the Fatah al-Sham group without hitting other rebels as well. Aleppo is Syria’s largest city and the new focus of the conflict.

The ultraconservative Ahrar al-Sham group, one of the leading factions in Aleppo, said rebel factions would release a unified position statement on the truce. Forty days of fighting in Aleppo has killed nearly 700 civilians, including 160 children, according to a Syrian human rights group.

In the past, several cease-fires were brokered—all which failed to hold. The Russian and U.S. governments contend this will go beyond several previous truces between the Syrian government and armed opposition.