Aden - Yemen’s Houthi rebels said Saturday that the U.S. military launched a series of airstrikes on the capital, Sanaa, and the Houthi-held coastal city of Hodeida, less than two days after a U.S. strike wrecked a Red Sea port and killed more than 70 people.

The Houthis’ media office said 13 U.S. airstrikes hit an airport and a port in Hodeida, on the Red Sea. The office also reported U.S strikes in the capital, Sanaa.

There were no immediate reports of casualties.

The U.S. military’s Central Command, which oversees American military operations in the Middle East, said it continues to conduct strikes against the Houthis in Yemen.

Thursday’s strike hit the port of Ras Isa, also in Hodeida province, killing 74 people and wounding 171 others, according to the Houthi-run health ministry. It was the deadliest strike in the U.S. ongoing bombing campaign on the Iranian-backed rebels.

United Nations Secretary-General António Guterres on Saturday said he was “gravely concerned” about the attack on Ras Isa, as well as the Houthis missile and drone attacks on Israel and the shipping routes, his spokesman Stéphane Dujarric said Saturday.

“The secretary-general recalls that international law, including international humanitarian law as applicable, must be respected at all times, and he appeals to all to respect and protect civilians as well as civilian infrastructure,” Dujarric said.

U.S. Central Command declined to answer any questions about possible civilian casualties. It referred to a statement in which it said “this strike was not intended to harm the people of Yemen.”

The strikes on Hodeida have been part of a month-long U.S. bombing campaign, which the Trump administration said came about because of the Houthi attacks on shipping in the Red Sea, a crucial global trade route, and on its close ally, Israel.

About 200 people have been killed in the U.S. campaign since March 16, according to the Houthis’ health ministry.