"Regime warplanes launched their first air strikes on the town of Khan Sheikhun in Idlib's southern countryside" since late Thursday, the Britain-based Syrian Observatory for Human Rights reported.

The bombardment came minutes after Syria's army said it would resume operations against the Idlib region, just days after it agreed to a truce ending months of deadly bombardment.

"Armed terrorist groups, backed by Turkey, refused to abide by the ceasefire and launched many attacks on civilians in surrounding areas," state news agency SANA reported Syria's military as saying, referring to jihadists and rebels.

"The armed forces will resume their military operations against terrorists," it said, ending a truce that came into effect on Friday.

Minutes later, Damascus said rebel fire hit near a key Russian air base.

"Terrorist groups targeted the Hmeimim air base with a flurry of rockets that fell near the airbase and caused great human and material losses," SANA reported a military source as saying.

Most of Idlib province and parts of Hama, Aleppo and Latakia -- which currently hosts some three million residents -- are controlled by former Al-Qaeda affiliate Hayat Tahrir al-Sham (HTS).

The region is supposed to be protected from a massive government offensive by a Turkish-Russian buffer-zone deal that was reached in September last year, but it has come under increasing fire by Damascus and its backer Moscow since the end of April.

The government of Syrian President Bashar al-Assad has accused Turkey of dragging its feet in implementing the deal, which provided for a buffer zone of up to 20 kilometres (12 miles) between the two sides, free of heavy and medium-sized weaponry.

On Monday, the Syrian army said it would resume bombardment because last week's truce had been "conditional" on Ankara implementing the buffer zone, according to SANA.

It accused Ankara of "failing to meet its obligations" by allowing armed groups to continue carrying out attacks, SANA said.

Air strikes on the Idlib region had stopped on Friday after the government's truce announcement. Fighting since late April has killed 790 civilians in regime and Russian attacks, according to the Observatory.

Fighting over the same period has claimed the lives of nearly two thousand combatants, including 900 regime loyalists, the monitor says.

Around 400,000 people have been displaced and dozens of hospitals and schools damaged, according to the United Nations.

Syria's conflict has killed more than 370,000 people and driven millions from their homes since it started with the brutal repression of anti-government protests in 2011.