ADEN, Yemen, Oct. 7 (Xinhua) -- Clashes broke out between pro-government Yemeni forces and Houthi rebels in the southern province of Lahj early Friday, leaving 10 killed from both sides, a military official told Xinhua.

"The Houthi rebel group sent scores of its fighters to attack sites controlled by the pro-government southern forces in Lahj, sparking hours of intense clashes in the area," the local military source said on condition of anonymity.

The government forces killed six Houthi attackers and lost four of their soldiers, he added.

Having failed to make military progress on the ground, the Houthis are now forced to retreat from key sites around the Yafea area near the rebel-held province of al-Bayda, according to the source.

The clashes come just days after the failure to extend a six-month truce brokered by the United Nations, as fighting has renewed between the Houthis and the pro-government forces in the southern regions.

Following the intense clashes, the pro-government forces of the Southern Transitional Council dispatched troops to fortify its sites in Yafea and other southern areas near the Houthi-held provinces, the military official told Xinhua.

On Sunday, the UN Special Envoy for Yemen Hans Grundberg announced that no agreement was reached to renew the expired truce between the Yemeni warring parties.

The six-month humanitarian truce between the Yemeni government and the Houthi militia went into force on April 2 and was later renewed twice through Oct. 2.

Yemen has been mired in a civil war since late 2014 when the Iran-backed Houthi militia seized control of several northern cities and forced the Saudi-backed Yemeni government out of the capital Sanaa.