JERUSALEM (AP) — Russian troops have arrived in Syria in recent days to aid Bashar Assad's beleaguered government in the battle against Islamic State militants, Israel's defense minister said Thursday. The development could help the Syrian president reverse his recent battlefield losses in the country's bitter civil war, now in its fifth year.

Moshe Yaalon said the Russians had dispatched military advisers as well as an active force, with the main goal of setting up an air base. The base, near the Syrian city of Latakia, could deploy fighter jets and helicopters in strikes against IS militants.

"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," Yaalon said in a briefing with Israeli reporters.

Yaalon said Russia's first goal was to protect its interests in Syria, namely its navy base there. He did not elaborate on how Israel knew of the Russian deployment in Syria or reveal the source of his information.

The claim was the latest indication of a Russian military buildup in Syria that has concerned the United States and NATO.

Moscow has backed Assad throughout the nation's civil war, which has killed more than 250,000 people. Russian President Vladimir Putin has sought to cast arms supplies to Assad's government as part of international efforts to combat the Islamic State group and other militant organizations in Syria.

But Putin has not ruled out a larger role and the move could mean that he has now decided to intervene directly on Assad's behalf. The U.S. and its allies see Assad as the cause of the Syrian crisis, and Washington has warned Moscow against beefing up its presence.