Yemen's Houthi rebel fighters launched a series of attacks on areas controlled by government forces in the southern regions during the past 24 hours, with eight killed from the two warring sides, a military said on Monday.

"Sites of the government forces particularly in the southern regions of Dhalea and Lahj were heavily attacked by the Houthis, sparking hours of intense armed confrontations during the past hours," the local military source said on condition of anonymity.

At least three soldiers were killed after a government forces' outpost in Dhalea was shelled by the Houthis, he added.

Elsewhere in southern Yemen, the Houthis launched a large-scale military operation against the government-controlled sites near the province of Lahj, sparking hours of intense battles with the forces stationed there.

"The government forces aborted the Houthi attack ... following the ferocious fighting," he said, adding at least five Houthi fighters were killed.

Since the failure to extend an UN-mediated humanitarian cease-fire between the Yemeni government forces backed by the Saudi Arabia-led coalition and the Houthis earlier this month, fighting between the warring sides has heavily intensified in various areas of the war-ravaged Arab country.

Hans Grundberg, the UN special envoy for Yemen, announced earlier this month that no agreement had been reached to renew the expired truce between Yemen's warring parties.

The nationwide truce went into force on April 2 and was renewed twice through Oct. 2.

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

The war has killed tens of thousands of people, displaced 4 million, and pushed the country to the brink of starvation.