Anti-government forces advanced Tuesday toward President Abedrabbo Mansour Hadi's refuge of Aden in southern Yemen, fighting fierce battles with loyalist forces in which at least 30 people were killed.

The Shiite Huthi militia, backed by troops allied to former leader Ali Abdullah Saleh, clashed with forces loyal to the president in at least two southern provinces as they pushed on Aden, sources said.

In another southern city, Taez, the militia shot dead five demonstrators as protests intensified against the Huthi presence.

The militiamen have seized control of large parts of Yemen and in recent days have been moving toward Aden, where Hadi fled after escaping house arrest in the capital Sanaa last month.

Yemen, a long-time US ally which borders Saudi Arabia, is increasingly divided between a north controlled by the Huthis, allegedly backed by Iran, and a south dominated by Hadi supporters.

The UN Security Council, Western countries and Gulf Arab monarchies have backed Hadi as the country's legitimate ruler and his foreign minister called on Monday for a Gulf intervention to confront the Huthis.

Tuesday's clashes came after the Huthis seized the airport and a nearby military base Sunday in Taez, 180 kilometres (110 miles) north of Aden and seen as a strategic entry point to Hadi's southern refuge.

Tribal sources said at least 30 people died late Monday as the Huthis and their allies clashed with armed tribesmen loyal to Hadi in the central Baida province and in Marib, east of Sanaa.