俄罗斯空袭美国支持的叙利亚反政府武装

Russian aircraft have dropped bombs on rebels battling Islamic State in southern Syria, including those supported by the United States, a senior U.S. official said.

The official, who asked not to be identified, said Russian jets had not been active in the location along the Syrian border with Iraq and Jordan, for some time, which raised “serious concerns” about the intentions of the Russians.

“We will seek an explanation from Russia on why it took this action and assurances this will not happen again," the official said Thursday.

In the past, the U.S. has been critical of the Russian presence in Syria, and has repeatedly refused to work with Russian forces in the country, which the U.S. accused of working with Syrian President Bashar al-Assad to consolidate his power.

The bombing Thursday is likely to test already strained relations between the U.S. and Russia, and came just a day after U.S. Secretary of State John Kerry warned Russia and Syria to respect a cease-fire treaty signed between the three nations earlier this year.

"Russia needs to understand that our patience is not infinite." Kerry said Wednesday following a meeting with Iranian officials.