The Saudi-led coalition said it launched a new military operation against Yemen’s Houthi rebels, in its first known strike since an attack on the kingdom’s oil industry that was blamed on Iran.

The Houthis on Friday, September 20 accused the coalition of a “serious escalation” and of endangering a truce around the key port city of Hodeidah.

“The intensive raids on Hodeidah are a serious escalation that could torpedo the Sweden agreement,” said one of the rebel leaders, Mohammed Abdessalem, referring to the U.N.-supervised truce agreed in December.

The operation in the north of the war-battered country comes after strikes on two Saudi oil facilities last weekend knocked out half the kingdom’s production, sending regional tensions soaring.

The coalition said its operation destroyed four sites north of Hodeidah that were used by the rebels to assemble remote-controlled boats and sea mines, according to a statement released by the official Saudi Press Agency.

It described the sites as a threat to maritime security in the crucial shipping lane of Bab al-Mandeb and the southern Red Sea.

Hours before announcing the operation, which strains the already fragile ceasefire in Hodeidah, the coalition said it intercepted a “remote-controlled, booby-trapped boat” that the rebels aimed to use for a “terrorist act in the south of Red Sea.”

The coalition said the boat was destroyed in the operation but it did not specify the target.

The coalition intervened in support of the Yemeni government in 2015 when President Abd Rabbo Mansour Hadi fled into Saudi exile as the rebels closed in on his last remaining territory in and around second city Aden.

The conflict has since killed tens of thousands of people – most of them civilians – and driven millions more to the brink of famine in what the United Nations calls the world’s worst humanitarian crisis.

The coalition, assisted by Western powers including the U.S., has struggled to oust the militia group that has stepped up attacks on Saudi cities.

The Houthi rebels claimed responsibility for last weekend’s attacks on Abqaiq, the world’s largest oil processing facility, and the Khurais oil field in eastern Saudi Arabia.

But both the U.S. and Saudi Arabia have ruled that out, saying the operation was beyond the Yemeni insurgents’ capabilities, and pinned the blame on Saudi Arabia’s arch rival Iran.

With reporting from AFP