U.S. officials have concluded that drone attacks on Saudi Arabia’s oil industry in May were launched from Iraq, not Yemen, raising concerns that Iran’s allies in the region are trying to open a new front in the conflict between Tehran and Washington.

Secretary of State Mike Pompeo urged Iraq’s prime minister to take steps to ensure that Iraq isn’t used as a new staging ground for attacks. Iraqi leaders are questioning the U.S. assessment and have asked the Trump administration for more evidence to support its claims.

The...