MOSCOW -- Russian Foreign Minister Sergey Lavrov on Friday called on world powers to help arm the Syrian army, describing it as the most efficient force against the Islamic State group.

The U.S. and NATO have raised concerns over Russia's military buildup in Syria since they see President Bashar Assad as the cause of the Syrian crisis, which has claimed more than 250,000 lives in over four years. Moscow in its turn has sought to cast arms supplies to Assad's government as part of international efforts to combat Islamic State militants.

The increased Russian activity in Syria reflects Moscow's deep concerns that its longtime ally is on the brink of collapse, as well as hopes by President Vladimir Putin that a common battle against Islamic State extremists can improve Russia's ties with the West, strained over Ukraine.

Lavrov said in Moscow on Friday that Russia would continue to supply Assad with weapons and called on other countries to help the Syrian government and its ground troops.

"You cannot defeat Islamic State with air strikes only," Lavrov said. "It's necessary to co-operate with ground troops and the Syrian army is the most efficient and powerful ground force to fight the IS."

Lavrov insisted that by sending weapons to Syria Russia is not propping up Assad but is contributing to defeating Islamic State fighters.

"I can only say once again that our servicemen and military experts are there to service Russian military hardware, to assist the Syrian army in using this hardware," he said at a presser in Moscow. "And we will continue to supply it to the Syrian government in order to ensure its proper combat readiness in its fight against terrorism."

Russia has been a longtime backer of Syria, and it has supported Assad throughout the civil war by shielding him from UN sanctions and providing weapons.

Putin is expected to focus on the situation in Syria when he addresses the UN General Assembly at the end of the month, and some analysts believe that Russia wants to have its military force in Syria ready for action by the time he speaks.