The world's supply of oil hit a record last month, yet another sign of oil's dominance over the energy landscape.

The International Energy Agency said Thursday that the global oil supply reached 100 million barrels a day for the first time ever, boosted by rising production in the U.S. and several OPEC nations.

Demand for oil grew more slowly in the second quarter but will continue to rise this year and in 2019, the group said.

Much of the increase in production is coming from OPEC and the United States, according to IEA, which advises oil-consuming nations.

U.S. production has returned to 201...