Fierce clashes between rebels and pro-government forces killed dozens across south Yemen on Saturday, threatening to derail a humanitarian ceasefire drawn up to bring vital aid to the war-wracked country.
The five-day truce initiated by a Saudi-led coalition that has bombarded the Iran-backed rebels for more than six weeks expires late Sunday, and Riyadh has already warned it was "ready to act" against any ceasefire violations.
In the latest violence, at least 12 civilians were killed and 51 wounded when the Shiite Huthi rebels shelled several neighbourhoods in Yemen's third city Taez, military and local sources said.
The clashes came after overnight fighting killed 26 rebels and militiamen loyal to former president Ali Abdullah Saleh as well as 14 pro-government forces, military sources said.
The United Nations has expressed deep concern about the civilian death toll from the Saudi-led bombing as well as the humanitarian impact of an air and sea blockade imposed by the coalition.
It says more than 1,500 people have died in the conflict since late March.
Some aid has trickled into Yemen since the pause in fighting, but residents of areas where clashes persist complain they remain without the most basic supplies.
The fighting in Taez overnight forced many to flee to the countryside.
"Humanitarian aid hasn't reached Taez, where we haven't received fuel, food or medical equipment," said a government official in the city.
However, UN refugee agency the UNHCR sent two planes loaded with aid to Sanaa on Saturday, airport sources said, while Red Cross and Doctors Without Borders (MSF) medical crews also flew in to the capital.
The United Nations has called for the Saudi-led coalition to simplify import inspections after warning that supplies were still blocked.
UN coordinator Johannes van der Klaauw warned that the inspections, introduced under an arms embargo slapped on the Huthi rebels last month, were hampering aid deliveries.
- Clashes despite truce -
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"The arms embargo and its inspection regime results in commercial goods, be it by air or by ship, no longer reaching the country," he said.
Saudi Foreign Minister Adel al-Jubeir has accused the Huthis of repeatedly violating ceasefire terms, but the rebels have pledged to honour the truce.
"We are hoping that the Huthis will abide by the terms of the ceasefire and stop their aggressive behaviour if they want the ceasefire to hold," he said.
But clashes rocked Aden on Saturday, an AFP correspondent said.
Heavy artillery, including tank shells, hit the city's northern sector where rebels and troops loyal to President Abedrabbo Mansour Hadi continue to fight over territory, including a main road into central Aden, military sources said.
West Aden was also hit by shelling, they added.
And in southern Daleh province, five Huthis were killed overnight when their convoy was ambushed, an official said.
The chaos in Yemen has been exploited by armed groups, including the country's branch of Al-Qaeda, which is viewed by the United States as the world's most dangerous.
A local official said 36 Yemen soldiers were kidnapped by suspected Al-Qaeda members overnight in the southern port of Mukalla.
The extremist group has controlled Mukalla, the capital of Yemen's vast desert Hadramawt province, since April and has for months claimed deadly attacks against Yemen's government-controlled armed forces.
The official said Al-Qaeda seized the soldiers late Friday after accusing them of supporting the Huthis.
In nearby Shabwa province, armed tribesmen took control of an oil-producing region after two days of clashes with rebel fighters, tribal and military sources said.
A conference on Sunday in Riyadh is set to bring rival Yemeni factions around the table in a bid to end the crisis, but the Huthis, who want talks to be held in Yemen, are boycotting the meeting.
Saudi Arabia has vowed to continue military action in Yemen until Hadi's government is restored.