The US says it is speeding up weapons deliveries to a Saudi-led coalition bombing Houthi rebels who have taken up arms against the government in Yemen.
The US is also boosting intelligence sharing with the coalition, Deputy Secretary of State Antony Blinken said.
Aid agencies have warned of a looming humanitarian crisis in the Yemeni port of Aden, which has seen street battles between rebels and government allies.
Some 550 people are said to have died in the last fortnight of fighting.
According to the World Health Organization, nearly 2,000 people have been injured over the same period.
The UN children's agency, Unicef, says at least 74 of the dead are children, and more than 100,000 people have been displaced.
A spokeswoman for the International Committee of the Red Cross said the situation in Aden was "catastrophic to say the least".
"The war in Aden is on every street, in every corner," Marie Claire Feghali told the AFP news agency. "Many are unable to escape."
The Red Cross is one of several aid agencies trying to send more medical supplies and workers to Aden.
The port has been largely cut off since the Houthi rebels and their allies advanced upon it in March, forcing President Abdrabbuh Mansour Hadi to flee to Saudi Arabia.
Fighting escalated in the city this week, with reports of overflowing hospitals, hijacked ambulances and bodies left in the streets.
On a visit to the Saudi capital, Riyadh, on Tuesday, Mr Blinken said the US had "expedited" the delivery of weapons to the Saudi-led coalition.
He said the move would support Saudi Arabia, which was "sending a strong message to the Houthis and their allies that they cannot overrun Yemen by force."
He said the US had also boosted intelligence sharing with the coalition and set up a centre to coordinate with Saudi operations.
On Tuesday, aircraft from the Saudi-led coalition reportedly bombed a military base in central Yemen, near the city of Ibb.
The raid targeted forces loyal to former President Ali Abdullah Saleh, who are fighting alongside the Houthis. The strike apparently took place near a school, and a rebel television station said three children were killed.
A Saudi official meanwhile denied reports from Aden that the city had been shelled by ships belonging to the coalition.
Earlier, the head of the Red Cross (ICRC) in Aden, Robert Ghosen, told the BBC the port had become a "ghost city".
"We are seeing a lot of people arriving dead at the hospital or dying in the hospitals," he told the BBC's Today programme. "The hospitals don't have the right supplies and the right staff."
Nizma Alozebi, a student from Aden, told the BBC that the violence had spread to residential areas and most shops.
"People are afraid for their belongings and their safety. It's insanity," she said.
President Hadi was forced to flee Yemen two weeks ago, as the rebels advanced southwards from the capital, Sanaa.
The Houthis have said their aim is to replace his government, which they accuse of being corrupt.
Saudi Arabia says the Houthis have military backing from regional rival Iran, which denies the allegation.
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
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