Yemen has been devastated by a civil war that began in 2014, when the Houthis ousted the government from Sanaa. The conflict escalated in 2015, after the Saudi-led coalition intervened in an attempt to restore the government's rule.

A four-year-old informal ceasefire began to unravel in July, when the Houthis announced a "maritime embargo" against Saudi Arabia and began missile and drone attacks on Saudi airports, oil facilities and tankers in the Red Sea.

The Houthis said they were retaliating for what they say is a Saudi blockade of ports and airports in their territory, as well as an air strike on Sanaa airport that they blamed on the kingdom. The Yemeni government claimed it was behind that strike, saying it was trying to prevent an unauthorised Iranian plane from landing.

Last month, the Houthis launched a surprise offensive along Yemen's south-western Red Sea coast and ousted pro-government forces from areas overlooking the Bab al-Mandab Strait, at the southern end of the Red Sea.

They said they posed no threat to international shipping passing through the waterway but that they would continue to target vessels from Saudi Arabia.

However, the seizure of the strait contributed to a surge in global oil prices because Saudi Arabia had been relying on the Red Sea for oil exports after the US and Israel's war with Iran effectively closed the Strait of Hormuz in the Gulf.

Last week, the Yemeni government announced the start of a new offensive aimed at retaking all territory held by the Houthis.

Pro-government forces said they were quickly able to retake areas around Bab al-Mandab with the help of intense Saudi air strikes, but the Houthis denied losing any territory.