Humanitarian agencies said Tuesday they could not safely get aid into conflict-hit Yemen and that a local Red Crescent worker had been shot dead while evacuating the wounded.
"The closure of all the international airports in Sanaa, Aden and Hodeida, and heavy restrictions on the seaports, are hampering the delivery of humanitarian assistance," Doctors Without Borders said in a statement.
As Arab warplanes pounded the country for a sixth day, the medical charity, known by its French acronym MSF, said it had already treated hundreds of wounded.
But it warned the "current inability to send in medical supplies and trained personnel means the situation is reaching a critical point."
MSF director of operations Greg Elder said "even reaching the areas of greatest need within Yemen is difficult, as domestic flights have been cancelled and movements inside the country are risky."
The organisation said it had received more than 550 patients at its emergency surgical unit in the southern city of Aden since March 19 as a result of fighting.
"We must provide support to our field teams who are exhausted and overloaded," Elder said.
The International Committee of the Red Cross said it could not fly in desperately-needed aid due to a lack of security.
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It also called for humanitarian workers be allowed to work safely in Yemen, after Red Crescent volunteer Omar Ali Hassam was shot dead Monday in the southern province of Al Dhalea while evacuating the wounded.
The ICRC said it had a plane stocked with medical supplies to treat up to 1,000 people that had been due to arrive in the country Tuesday but efforts to negotiate its safe arrival had so far failed.
The supplies were "for distribution to hospitals across the country that are running low on the means to treat the war wounded" after a week of deadly clashes and air strikes.
"There are casualties across the country. There have been air strikes in the north, west and south, and clashes between opposing Yemeni armed groups in the centre and south, that are putting immense strain on already weak medical services," said Cedric Schweizer, who heads an ICRC team of 300 people in Yemen.
"In order that the wounded get the treatment they deserve, it's essential we deliver urgent medicines and surgical kits," he added.
An ICRC surgical team was due to arrive shortly in the southern city of Aden, which has suffered the biggest casualties so far, the organisation said.
A Saudi-led Arab coalition has been conducting air strikes against Huthi rebels in Yemen since Thursday.
It has vowed to keep up the raids until the Iran-backed rebels abandon their insurrection against President Abedrabbo Mansour Hadi, who has fled to Riyadh.