Fierce clashes raged between rebels and loyalists in southern Yemen, leaving more than 140 dead in 24 hours, as the Red Cross faced delays to urgently needed aid deliveries.
Relief workers have warned of a dire situation in the impoverished Arabian Peninsula state, where a Saudi-led coalition is waging an air war on the Iran-backed Huthi Shiite rebels.
The bloodiest fighting occurred between rebels and loyalists of President Abedrabbo Mansour Hadi in the main southern city of Aden, officials said.
Fifty-three people, more than a third of those slain in the past 24 hours, were killed as rebels tried to seize a port in the city, which sits on an extinct volcano jutting out into the sea.
At least 19 Huthis and 15 pro-Hadi militiamen were killed overnight in the town of Daleh, north of Aden, officials said, and seven other people died in the southern province of Abyan.
Pro-Hadi fighters have laid siege in Abyan to the base of a renegade army brigade loyal to ex-president Ali Abdullah Saleh, who is accused of backing the Huthis.
Ten rebels were killed in coalition air strikes that hit Al-Anad airbase near Abyan and a military camp in Lahj, while many more died in Shabwa province.
Hadi, considered by the UN to be Yemen's legitimate leader, took refuge in Aden in February after the Huthis, who hail from the mountainous north, seized power in Sanaa.
Hadi fled to Saudi Arabia last month as rebels advanced on his southern stronghold, prompting the military campaign by the Saudi-led coalition, now in its 12th day.
Yemen, strategically located near key shipping routes and bordering oil-rich Saudi Arabia, is sinking deeper into a multi-sided civil conflict.
The fighting has drawn in an array of armed groups including the Huthis, pro-Hadi militia, army units loyal to Saleh, southern separatists, Sunni tribes and Al-Qaeda militants.
- Aid flight 'problems' -
The Red Cross has appealed for an immediate truce to facilitate aid deliveries and has called for all land, air and sea routes to be immediately opened to allow in 48 tonnes of medical supplies the organisation has ready to treat up to 3,000 wounded.
The situation is particularly dire in Aden, where some neighbourhoods have had no electricity or water in days.
The Red Cross has been trying to fly emergency supplies into Sanaa but the plane is still stuck on the tarmac.
"We have a cargo plane with medical supplies which is ready to go," said Sitara Jabeen, a spokeswoman for the International Committee of the Red Cross.
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"We have the permission for this plane but we have logistical problems for the landing. There are less and less planes landing in Yemen. We are trying to solve the logistic problems," she told AFP.
The Red Cross is also trying to deploy a team of surgeons to Aden, but says it is still awaiting authorisations from all sides in the fighting.
But the spokesman of the Saudi-led coalition, General Ahmed Assiri, said the Red Cross had evacuated 11 staff members from Sanaa airport.
It was unclear if they were to be redeployed elsewhere in Yemen or replaced.
Russia has urged the UN Security Council to push for a humanitarian pause in the strikes waged by Saudi Arabia and a coalition of five Gulf monarchies, Egypt, Jordan, Morocco and Sudan.
- Pakistan standing by -
On Monday an Indian vessel evacuated 450 people from the western Hodeida port, while 100 others boarded a Chinese vessel to safety. France evacuated 63 foreigners from the eastern port of Balhaf.
Jordan said it had evacuated almost 300 nationals since the conflict began and an AFP photographer said three Indian aircraft and one Russian plane landed Monday in Sanaa to evacuate their citizens.
Meanwhile the United States admitted it has no immediate plans to rescue its nationals stranded in Yemen and urged them to flee the raging conflict by sea.
State Department spokeswoman Marie Harf could not confirm a claim that 200 American civilians are caught up in the conflict, but said US authorities would contact those that it could to advise them of opportunities to board ships sent by allied nations.
Pakistan said it had been asked by Saudi Arabia to contribute aircraft, ships and ground troops to Operation Decisive Storm.
Despite close ties with Riyadh, Pakistan has so far held back from joining the offensive, saying it does not want to get involved in any conflict that would inflame sectarian tensions.
Shiite Iran has strongly criticised the intervention in Yemen and has rejected accusations of arming the Huthis.
Its foreign minister, Mohammad Javad Zarif, is due to visit Islamabad on Wednesday to discuss the situation. Pakistan is holding a special session of parliament to debate any decision on joining the intervention.
In Riyadh, the Saudi cabinet reiterated the Sunni monarchy "is not seeking war" and that the offensive was to "rescue a neighbouring country... and its legitimate authority".