(Adds Islamic State preacher, background)
BEIRUT, Sept 24 (Reuters) - 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 on Thursday, in an attempt to break a siege on a nearby air base.
Russia is bolstering its ally Damascus against rebels with military aid that U.S. 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 only "recently" in Syria but were being flown by Syrian pilots, said the Observatory, which tracks Syria's conflict via a network of 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. But the rise of a common enemy, Islamic State, has rendered divisions less clear.
The United States 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 United States 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 U.S.-led coalition that is operating out of neighbouring Turkey has also hit IS targets in Raqqa. The United States has warned Syria not to interfere in its air campaign.
A mosque preacher in Raqqa spoke on Thursday 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 exact scale and scope of its military presence in Syria.
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. (Reporting by John Davison; Editing by Mark Heinrich)
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