Russian forces have arrived to aid President Bashar Assad's beleaguered government, Israel's defence minister has said.
Moshe Yaalon told reporters that Moscow sent military advisers and an active force to set up an airbase. He says the base, near the Syrian city of Latakia, could be used to deploy fighter jets and helicopters in strikes against Islamic State militants.
Mr Yaalon's comments support concerns from the US and Nato that Russia is increasing its military presence in Syria.
Moscow has backed Assad throughout Syria's four-and-a-half-year civil war, in which more than 250,000 people have been killed.
Mr Yaalon told reporters: "As far as we understand, at this stage we are talking about a limited force that includes advisers, a security team and preparations for operating planes and combat helicopters."
He said Russia's first goal was to protect its interests in Syria, namely its navy base there.
Mr Yaalon described the Russian move as "significant", and said if Russia plans on carrying out airstrikes against IS militants, they would have to coordinate it with a US-led campaign.
Earlier, Russia's foreign minister Sergey Lavrov said Russian aircraft flying into Syria have been delivering military supplies and humanitarian aid.
He confirmed that Russia has military personnel in Syria for training purposes, but said nothing about reported deployments of additional troops.
Asked about Russian planes flying to the airfield near Latakia, Mr Lavrov said they were carrying "military goods in accordance with existing contracts and humanitarian aid".
When asked if Russia could deploy its troops to Syria to help fight IS, Russia's President Vladimir Putin said last week that he was "looking at various options".
Mr Putin is expected to focus on the situation in Syria when he addresses the UN General Assembly at the end of the month.
Mr Assad's forces have recently suffered a series of setbacks on the ground.
So far, two out of Syria's 14 provinces are out of government control - Idlib, which fell Wednesday to al-Qaida militants and other Islamic fighters, and Raqqa, which was seized in 2014 by the Islamic State. The IS has declared a caliphate on the territory it controls in Syria and Iraq.