The Red Cross has called for a 24-hour ceasefire in Yemen to bring in desperately needed medical supplies after fierce fighting.
It says medical teams and rescuers must access Aden, a stronghold of those loyal to President Abdrabbuh Mansour Hadi, or more civilians will die.
Fighting in the southern port city has intensified with Saudi-led airstrikes aiming to push back Shia Houthi rebels.
Russia, too, has called at the United Nations for a truce.
It presented a draft resolution to the Security Council in New York, where an emergency meeting was scheduled, calling for a “humanitarian pause”.
The Red Cross said the streets of Aden were “strewn with dead bodies” and the wounded were “streaming” into hospitals and clinics.
Over the past two weeks, fighting in Yemen has left more than 500 people dead and some 1,700 wounded, UN humanitarian chief Valerie Amos has said.
Saudi warplanes and ships have bombarded rebel positions for 10 days.
“Our relief supplies and surgical personnel must be allowed to enter the country and safely reach the worst-affected places to provide help,” said Robert Mardini head of operations for the International Committee of the Red Cross in the Middle East.
“Otherwise, put starkly, many more people will die.
“For the wounded, their chances of survival depend on action within hours, not days,” he added.
The Red Cross said it had 48 tonnes of medical equipment to treat up to 3,000 people ready to leave, pending clearance.
Britain’s deputy UN ambassador Peter Wilson expressed regret for civilian casualties but said Britain would carry on supporting Saudi-led military action against the Houthi rebels.
President Hadi fled to Saudi Arabia on 25 March after the rebel forces advanced on Aden.
He faces opposition from troops loyal to the former president Ali Abdullah Saleh, ousted in the Arab Spring protests, and the Houthis.
Also ranged against President Hadi is al-Qaeda in the Arabian Peninsula which opposes the Houthis too.
This week, the Shia Houthi rebels pushed through to the heart of Aden using tanks and armoured vehicles. But with Saudi assistance including airdrops of weapons, they were forced back from the presidential palace.
Meanwhile, armed tribesmen pledging to restore security have entered the south-eastern city of Mukalla taken over by al-Qaeda militants at the end of last week, Reuters news agency reported.
The Houthis: Zaidi Shia-led rebels from the north, who seized control of Sanaa last year and have since been expanding their control
President Hadi: Fled to Saudi Arabia after rebel forces advanced on his stronghold in the southern city of Aden
Al-Qaeda in the Arabian Peninsula: Seen by the US as the most dangerous offshoot of al-Qaeda, AQAP opposes both the Houthis and President Hadi.
Islamic State: A Yemeni affiliate of IS has recently emerged, which seeks to eclipse AQAP