BEIRUT (Reuters) - Air strikes hit a part of northwest Syria for the first time since a ceasefire in the region was declared 10 days ago, a war monitor and rebel spokesman said on Tuesday.

Russia denied that its forces, which back Syrian President Bashar al-Assad’s government in the eight-year-old conflict, had carried out any such attacks.

The government and Russia unilaterally agreed to a truce on Aug. 31 in opposition-controlled Idlib, where a “de-escalation zone” was brokered two years ago.

Since then, the intense air strikes by Russian and Syrian warplanes that had accompanied a Syrian government push to re-take the area have stopped, although there has been ground fighting and shelling.

The United States said its forces had carried out strikes against an al-Qaeda facility in Idlib on the day the ceasefire came into effect.

The Syrian Observatory for Human Rights war monitor, based in Britain, said planes had carried out two raids in the strategic Jabal al-Akrad mountain range near the western Latakia coast.

It was not clear if these raids signal a return to the Russian and Syrian campaign of heavy air strikes.

The Russian Defence Ministry denied that Russian or Syrian warplanes struck the Idlib de-escalation zone.

“Russian and Syrian air forces have not been carrying out any military missions to hit ground targets,” a statement carried by RIA news agency said.

Mohammad Rashid, spokesman for the Jaish al-Nasr rebel faction, said the two raids, which he believed were carried out by Russian planes, were the first since the ceasefire began.

The truce was the second declared in August in Idlib, the only major swathe of the country still in rebel hands after more than eight years of war. A ceasefire in early August collapsed three days in, after which the Russian-backed army pressed its offensive and gained ground.

Idlib province houses millions of people who have fled war elsewhere in Syria. Hundreds of civilians have been killed since an offensive to take the area began in April.

The dominant force in Idlib is Hayat Tahrir al-Sham, a jihadist alliance formerly known as the Nusra Front, which cut ties to al Qaeda in 2016. However, a wide array of factions, including Turkish-backed rebels, also have a presence.