Two tribesmen were killed in an ambush by Shiite Huthi militiamen in oil-rich Marib, a province in eastern Yemen on which the expanding movement has set its sights, a tribal source said.

The ambush took place in Naqil al-Watada, an area between Marib and Sanaa, the source said, adding that six armed tribesmen were wounded in a clash following the attack.

Several Huthis were also killed, the source said, without giving an exact toll for the Shiite movement which rarely discloses its losses.

The fighting came a day after the Huthis extracted major political concessions from President Abdrabuh Mansur Hadi, following escalating violence in Sanaa.

The Shiites' leader, Abdul Malik al-Huthi, threatened earlier this month to take control of Marib, which supplies electricity and fuel to most of Yemen including the capital.

And in a televised speech on Tuesday after his men seized the presidential palace in Sanaa, Huthi demanded that authorities "immediately" improve the security situation in Marib, where Sunni tribes have been mobilised to resist Huthi advances.

Al-Qaeda militants, who have fought the Huthis alongside the Sunni tribes, also have a presence in Marib province.

State news agency Saba on Thursday, quoting tribal dignitaries and security officials in Marib, said local tribes were to aid security forces in protecting Marib's oil and gas installations, which are the target of frequent sabotage attacks.

The Huthis overran Sanaa on September 21 and have since been advancing in other regions of Yemen, sparking deadly clashes with Sunni tribes as well as Al-Qaeda.