Iran-aligned Houthis seized control of Yemen's port city of Mocha on Thursday and advanced down the Red Sea coast to strategic islands, military sources said, hours after President Donald Trump said he expected the Iran war to end after midterm elections.
Yemeni government military sources said the Houthis had gained further leverage over the Bab el-Mandeb Strait, the southern outlet of the Red Sea and one of the world's most important shipping routes, and had reached the islands of Hanish.
Red Sea navigation is safe for all shipping companies except for Saudi vessels, Yemen's Houthi-run humanitarian operations coordination centre said in a statement.
Saudi Arabia, the world's largest oil exporter, has relied on the Red Sea shipping route since the Iran conflict effectively closed the Strait of Hormuz, through which a fifth of global oil used to flow.
Houthi spokesperson Mohammed Abdulsalam said, without elaborating, that operations were defensive and would stop once attacks on Yemen stopped.
If the Houthis were to impose a stranglehold over the waterway, on the opposite side of the Arabian peninsula from the Strait of Hormuz, it could give their sponsor Iran a critical advantage in the war with the US, reducing supplies through a second major transit corridor and sending oil prices surging.
Brent crude prices jumped as much as 4% on Thursday to $105 a barrel on concern over disruption to supply routes.
The Houthis entered the current war alongside Iran with an attack on Israel in March, but largely stayed quiet before declaring a naval blockade against Saudi Arabia in July and stepping up attacks on the south of the kingdom this month.
Tensions in the region were high on Thursday, with Saudi civil defence authorities issuing emergency alerts in the southwestern city of Khamis Mushait for the fourth time in 24 hours, as the Houthis attacked.
Pakistan has delivered a Saudi warning to Iran to rein in its Yemeni Houthi allies in an effort to prevent the crisis from spiralling, Saudi, Pakistani and Iranian sources said.
A senior Iranian official confirmed that Pakistan conveyed the message to Tehran on Tuesday and its response was that "Iran does not control the Houthis".
The conflict in Yemen between Saudi Arabia's Yemeni allies in the internationally backed government and the Houthis, comes as Iran and the United States staged their biggest declared wave of tit-for-tat attacks on shipping in the Gulf region this week since the war began in February.
Seen as part of a regional alliance known as the "Axis of Resistance" backed by Iran, the Houthis have also rallied behind the Palestinians in the Gaza war, lobbing drones and missiles at Israel and attacking commercial shipping in the Red Sea.
Yemeni government forces and their allies are relocating south along the Red Sea coast to Dhubab, which sits directly on the strait, across from the island of Perim, government military sources said. Control of Dhubab and the island is key to gaining hold of the strait, they said.
Bab el-Mandeb, or the Gate of Tears, named for its perilous navigation conditions, lies between Yemen on the Arabian Peninsula and Djibouti and Eritrea on the African coast, making it a major prize for the Houthis, who control Yemen's most populous areas and much of the north.
It is one of the world's most important routes for crude oil and fuel from the Gulf bound for the Mediterranean via the Suez Canal, or the Suez-Mediterranean pipeline on Egypt's Red Sea coast, as well as commodities bound for Asia, including Russian oil.