Strikes by Yemen's Houthis on Sunday killed at least 11 people, a medical source in the Yemeni port city of Mokha has told AFP, after an earlier drone salvo targeted a Saudi oil refinery on the Red Sea coast.

A years-long truce in Yemen's civil war between the Houthis and the Saudi-backed internationally recognised government appeared to collapse last month.

It came as the Houthis announced a maritime blockade of Saudi Arabia, hitting Saudi tankers in the Red Sea and the kingdom's oil infrastructure, in parallel with Iran's blockade of the Strait of Hormuz.

A medical source in Mokha, held by Yemen's internationally recognised, Aden-based government, said three civilians and eight military personnel had been killed in the Houthi attacks.

A further 32 individuals were wounded, six of them civilians, the source added, explaining all the civilian casualties had been a result of strikes on Mokha's port.