Syria began burying its dead Friday after a drone attack on a military academy graduation ceremony in Homs killed dozens, while Damascus pummelled opposition-held areas in response to the assault by "terrorist organizations."

In one of the bloodiest single attacks on the army since the start of Syria's war in 2011, Thursday's assault came just after the ceremony attended by officers and their families, killing and wounding both military personnel and civilians.

State media said Friday that 89 had died, including 31 women and five children, with 277 other people wounded. Dozens of distraught victims' relatives gathered outside the Homs military hospital since the early morning, an AFP correspondent said.

A woman in a black dress adorned with white flowers was overwhelmed with grief at the loss of her son. "Do not go, my beloved," she cried. "This sleep does not befit you."

The Syrian Observatory for Human Rights, a Britain-based monitor, reported a heavier toll of 123 dead, including 54 civilians, 39 of them women and children. It said at least 150 people were wounded.

In a rare move since the war broke out, the government has declared three days of mourning starting Friday, with flags flying at half-mast Defense Minister Ali Mahmoud Abbas attended the first funerals for around 30 people, including both military and civilians.

Military personnel saluted as rows of caskets draped in Syrian flags were carried one by one and placed into ambulances for transportation for burial.

Videos circulating on social media have shown panic and chaos during the attack, with people falling to the ground and others pleading for help.

"Martyrdom, dignity and national pride come at a great cost," Abbas told victims' families, according to a statement broadcast on state television.

The blood of those who died "is dear, but the nation is dearer," he added.

Abbas had been present at the graduation ceremony but left just minutes before the attack, eyewitnesses and the Observatory said.

At the military hospital, Khawlah, 33, was searching among the coffins for her brother.

"Amjad did not die, I died," she told AFP, grief-stricken.

No group has claimed responsibility for the assault, but the Syrian army has accused "armed terrorist organisations" of the attack with "explosive-laden drones", vowing to "respond with full force."

The Syrian army on Thursday began a bombing campaign on opposition-held areas in the country's northwest in apparent retaliation.

The Observatory said Friday that 15 civilians had been killed, adding that Russian warplanes carried out at least five strikes in the Idlib area.

Russian President Vladimir Putin condemned the academy attack and expressed his condolences to his Syrian counterpart Bashar al-Assad, the Kremlin said Friday.