Syrian government forces used newly arrived Russian warplanes to bombard Islamic State insurgents in Aleppo province in northern Syria, a group monitoring the civil war said yesterday, in an attempt to break a siege on a nearby air base. Russia is bolstering its ally Damascus against rebels with military aid that US officials say has included fighter jets, helicopter gunships, artillery and ground forces.

The air strikes, which began earlier this week, were accompanied by ground attacks near the Kweiris air base in the east of Aleppo province, where government troops have long been surrounded by Islamist militants, the Britain-based Syrian Observatory for Human Rights said.

The Russian jets arrived “recently” in Syria but were being flown by Syrian pilots, said the Observatory, which tracks Syria’s conflict via sources on the ground. Many Western countries have reacted with alarm to Moscow’s increased military support for President Bashar al-Assad, whom they oppose.

Assad wishes a blessed Eidal-Adha to Syrians facing death and destruction

But the rise of a common enemy, Islamic State, has rendered divisions less clear. The US is waging its own aerial campaign against the militants in Syria and Iraq. Stronger Russian involvement in Syria means that forces of the former Cold War foes could encounter each other as they battle the same enemy.

Russia says Assad, its closest Middle East ally, should be part of international efforts to fight Islamic State, while the US believes he is part of the problem.

Syrian government forces have stepped up air strikes against Islamic State in areas it controls since the arrival of fresh Russian military hardware, including in Raqqa, the jihadist group’s de facto capital in Syria. The US-led coalition that is operating out of neighbouring Turkey has also hit IS targets in Raqqa.

The US has warned Syria not to interfere in its air campaign. A mosque preacher in Raqqa spoke yesterday about the Russian “intervention”, the Observatory said, welcoming it as a chance for IS militants to fight against Russian forces. Russia has confirmed it has “experts” on the ground, but has declined to comment on the scale and scope of its military presence in Syria.

Syria’s President Bashar al-Assad (front, second from left) attends prayers on the first day of Eid al- Adha at al-Adel mosque in Damascus, Syria, yesterday.

Moscow’s increased support also comes after advances against Assad’s forces this year by non-Islamic State insurgents, including the government’s recent loss of a major air base and last bastion in the northwestern province of Idlib.

Meanwhile yesterday Assad prayed alongside government and religious officials in a Damascus mosque, making a rare public appearance for the Muslim Eid al-Adha holiday. Footage broadcast by state TV showed Assad sitting listening to a sermon. He addressed Syrians during a brief outdoor interview afterwards, in a video posted on social media.

"A blessed Eid al-Adha to all Syrians, who are facing death and destruction," Assad said, and praised soldiers fighting "terrorists" – the government's term for all insurgents battling against it in a four-year-old civil war.