BEIRUT— American-led coalition warplanes destroyed four tanks belonging to the Islamic State group in eastern Syria on Friday, the U.S. military said, targeting the militants in a key oil-producing area that has generated millions of dollars in revenue for the extremist faction.

The strikes outside the city of Deir el-Zour on the Euphrates River marked the second consecutive day that the United States and its Arab allies have taken aim at the militants near the border with Iraq. Coalition planes pounded a dozen makeshift oil-producing facilities in the same area on Thursday, trying to cripple one of the militants' primary sources of cash — black market oil sales that the U.S. says produce up to $2 million a day.