President Donald Trump offered U.S. support for Saudi Arabia’s self-defense in a call on Saturday with Saudi Crown Prince Mohammed Bin Salman, the White House said.

The pair spoke after a swarm of explosive drones hit key oil-industry targets in Saudi Arabia. The state-run Saudi Press Agency earlier reported on the call.

Iranian-backed Houthi rebels in Yemen claimed responsibility for the strikes, which forced Saudi Aramco to cut daily oil production in half.

The U.S. “strongly condemns today’s attack on critical energy infrastructure,” White House spokesman Judd Deere said in an emailed statement that was also posted on Twitter. The U.S. government “is monitoring the situation and remains committed to ensuring global oil markets are stable and well supplied.”

Republican Senator Lindsey Graham, a confidant of Trump, earlier urged a decisive U.S. response against Iranian targets.

“It is now time for the U.S. to put on the table an attack on Iranian oil refineries if they continue their provocations or increase nuclear enrichment,” Graham of South Carolina said on Twitter. “Iran will not stop their misbehavior until the consequences become more real, like attacking their refineries.”

John Abizaid, the U.S. ambassador to Saudi Arabia, said in a tweet “attacks against critical infrastructure endanger civilians, are unacceptable, and sooner or later will result in innocent lives being lost.”

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