Yemen agreed a truce with Houthis in February to halt a war that has raged on and off since 2004 and displaced 350,000 people.

But instability still threatens Yemen, neighbour to top oil exporter Saudi Arabia, which was briefly drawn into the conflict last year when rebels seized Saudi border areas.

Houthi rebels complain of socio-economic and religious discrimination by the government, accusations it denies.

Tensions between the rebels and the Ibn Aziz tribe, from the same Zaidi sect of Shia Islam but which sides with the government, have increased in past months.

Clashes last week were the bloodiest since the truce and drew in government forces.

Al-Qaeda hunt

Separately, security forces in southern Yemen killed three men believed to be al-Qaeda members in continuing security sweeps following the group's recent attacks on state security forces, a local security official said on Monday.

Ahmed al-Maqdisi, head of security in the southern province of Shabwa, said the three men were suspected of participating in an attack on Thursday on a security convoy in Shabwa that killed five soldiers and was blamed on al-Qaeda.

Al-Qaeda's regional wing based in Yemen had previously focused its high-impact strikes on foreign targets, but has started to also aim them at government forces in response to a recent US-backed crackdown on al-Qaeda.

Western powers fear the group could be exploiting growing instability in Yemen to use the country as a launchpad for attacks abroad.

Yemen also faces a separatist movement in its south and is under Western and Saudi pressure to quell domestic conflicts in order to focus on al-Qaeda.

On Sunday, gunmen killed six Yemeni soldiers in Shabwa, an oil-producing province.

It was the fourth assault since June attributed to al-Qaeda's regional Yemen-based arm that hit state targets, including intelligence and police offices.