Yemen’s Tehran-aligned Houthi movement has threatened to besiege a second key Middle East waterway as Iran launched drone attacks against US Navy vessels in the Strait of Hormuz over the weekend after a diplomatic effort to end the war appeared to collapse.

A closure of the Bab el-Mandeb Strait, which connects the Red Sea to the Indian Ocean, would effectively sever Saudi Arabia’s only remaining oil export route.

The kingdom has been shipping up to 7 million barrels of crude per day through the Red Sea port of Yanbu since its main Gulf export terminal at Ras Tanura – the world’s largest – was shut down by the Iran conflict.

The threats and clashes marked a dramatic deterioration in the war that the United States and Israel launched on February 28. They also raised the prospect of simultaneous closures of two of the world’s most critical maritime chokepoints.

It came on the heels of a frenetic weekend of updates. Late on Friday, Iran’s Foreign Minister Abbas Araghchi had declared that the Strait of Hormuz was open to commercial shipping, in what appeared to be a gesture towards renewed diplomacy.