The Red Cross says its staff in Sri Lanka are witnessing an "unimaginable humanitarian catastrophe" in the area where troops have trapped Tamil Tigers.
The agency says a ferry loaded with aid has been unable to reach the battered north-eastern coastal strip for three days because of fighting.
There are also reports that staff have quit the last hospital in the war zone.
A senior UN envoy is on his way to Sri Lanka to try "to help resolve the humanitarian situation", the UN says.
Vijay Nambiar, Secretary General Ban Ki-moon's chief of staff, is expected to arrive on Friday.
¥í¥ì¥Ã¥¯¥¹•rÓ‹¥«¥ë¥Æ¥£¥¨•rÓ‹IWC•rÓ‹¥°¥Ã¥Á•rÓ‹¥ª¥á¥¬•rÓ‹¥Ñ¥Í¥é¥¤•rÓ‹¥í¥ì¥Ã¥¯¥¹¥ì¥×¥ê¥«¥«¥ë¥Æ¥£¥¨¥³¥Ô©`¥í¥ì¥Ã¥¯¥¹¥³¥Ô©`¥°¥Ã¥Á¥³¥Ô©`¥Ñ¥Í¥é¥¤¥³¥Ô©`Last month Mr Nambiar met Sri Lankan President Mahinda Rajapaksa but failed to secure access to the war zone for humanitarian teams.
See a map of the conflict region
The UN says about 50,000 civilians are trapped in the war zone, although Colombo disputes this figure.
The government has rejected international calls to stop its offensive against the Tamil Tiger rebels, saying it would give them time to recover. Now that it has trapped the Tamil Tigers, it hopes to soon end the 25-year-old civil war.
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Sri Lanka warned on war crimes
The International Committee of the Red Cross (ICRC) said its chartered ferry, the Green Ocean, had been unable to deliver aid or evacuate the wounded for three days.
"Our staff are witnessing an unimaginable humanitarian catastrophe," said ICRC director of operations Pierre Krahenbuhl, based in Geneva.
cmc tube socketswbt tube sockets cmc binding postswbt binding postscmc banana & spadeswbt banana & spadescmc¡¡loudspeakerwbt loudspeaker"Despite high-level assurances, the lack of security on the ground means that our sea operations continue to be stalled, and this is unacceptable," he said.
"People are left to their own devices."
The ICRC says it requires security and unimpeded access to the area immediately.
In a statement, it said another aid ship, from the World Food Programme, was also waiting to deliver supplies to the war zone.
