Heavy fighting broke out around a Syrian military base on Friday, the Syrian Observatory for Human Rights said, in the first major violation of a ceasefire marking the Muslim holiday of Eid al-Adha.

The Observatory said rebel fighters were trying to storm the base, which is less than 1 km from the main north-south highway linking Damascus to Aleppo, and President Bashar Assad's forces had fired artillery at a nearby village.

On Thursday, Syria's military announced that it would respect a holiday ceasefire promoted by the international envoy to the Syrian crisis, adding, however, that it retains the right to retaliate to any violation to the truce by opposition forces.

According to the Syrian military, the ceasefire will go into effect on Friday morning. Syrian rebel forces, however, reported that they would honor the ceasefire only if prisoners held by Assad’s government were released.

The announcement, made on Syrian TV, came a day after Damascus' Foreign Ministry said that leaders of the Syrian army were studying a proposal by Lakhdar Brahimi for a cease-fire to mark the Islamic holiday of Eid al-Adha.

Speaking via a video linkup with members of the United Nations' Security Council, Brahimi said that the holiday ceasefire was an important step and could aid future attempts at ending the long-running crisis.