WASHINGTON — Russia warned in advance that its military would launch strikes Tuesday on Syria for the first time from a submarine stationed in the Mediterranean, the United States said.

The strikes were aimed at "two terrorist strongholds" around Raqa, the de facto Syrian capital of the Islamic State group, Russian Defense Minister Sergei Shoigu said.

Speaking in Washington, Pentagon spokesman Peter Cook said: "We were given advanced notice that they would be carrying out strikes."

He added: "We appreciate that," particularly because the Kremlin was not required to do so under a memorandum of understanding signed by Russia and the US to avoid the two sides clashing over Syria.

The advanced notice on the cruise missiles was "an additional measure of safety," said Cook.

The United States heads a coalition of some 60 countries taking part in air strikes on Syria and Iraq in a bid to destroy the self-proclaimed Islamic State group.

Moscow, a close ally of Syrian President Bashar al-Assad, is also waging an air war over Syria.