Even before the truce deadline passed at 11.59 p.m. local time (1059 UTC) on Sunday, it had already been unclear to what extent it would be observed by either side.

While witnesses and security officials told the AP news agency that Saudi-led coalition airstrikes had ceased, air raids had reportedly taken place in the 15 minutes leading up to the midnight truce deadline.

And although there was an apparent reduction in the level of violence in much of the country, in the southern province of Taiz - 125 kilometers (75 miles) northwest of Yemen's second city Aden - witnesses said Houthis shelled residential areas in the town of Jebel Sabr. There was also said to be fighting in the city of Taiz itself.

Fighting hotspots in Aden and Lahj - an outlying town to the north - were said to have become quieter after the ceasefire came into effect, although there were still occasional volleys of gunfire. An hour after the truce expired, the sound of gunfire in areas to the north of Aden was said by some witnesses to be "sporadic."

Houthis' rearguard action

Prior to the truce coming into place, the Houthi rebels had sought stem advances to the north of Aden by the forces local to exiled President Abedrabbo Mansour Hadi, who is supported by the Saudi-led Arab coalition. The rebels were then reported to have started to withdraw and begun regrouping at the country's largest military base.

The Saudi-led coalition announced the truce on Saturday, but stipulated that it reserved the right to respond to "military activity or movement."

Meanwhile, Houthi rebels have cast doubt on the coalition's humanitarian pause, with one official saying it could mark "the beginning of a new war," according to the AP news agency.

Head of the Houthi's Revolutionary Council, Mohammed Ali Al-Houthi, said Sunday that the group had not received official notification of the truce from the United Nations.

rc/lw (AFP, AP)