* 6,000 Asian and African migrants stranded in Misrata
* Rescue ship due to arrive soon but can only take 800
* IOM says needs more funds to complete evacuation
GENEVA, April 14 (Reuters) - A rescue ship is on its way to the besieged Libyan city of Misrata to begin evacuating 6,000 stranded migrants, many in an "extremely poor state", the International Organisation for Migration (IOM) said on Thursday.
But the chartered vessel can only carry 800 people at a time and current funds only cover the cost of two trips, the aid agency said.
The migrants have lived for weeks often in the open with limited food and no clean water in Libya's third largest city, the IOM said in a statement. Two-thirds are from Egypt with the rest from Niger, Bangladesh, Ghana, Sudan and Nigeria.
"It has been reported to IOM that the migrants are in a desperately worrying condition with many severely dehydrated and weak," the Geneva-based agency said.
The chartered vessel, which departed from the rebel-held eastern city of Benghazi on Wednesday, was due to reach Misrata later on Thursday. IOM officials on board would then begin the difficult task of choosing whom to evacuate first, it said.
"This operation is critical for saving lives of migrants because the situation is so bad. We can't take everyone at the same time," IOM spokeswoman Jemini Pandya told Reuters.
An IOM staff member on the board, who was not being named, was quoted as saying: "It is going to be heartbreaking not being able to take everyone at once."
"We should not be put in a position of deciding who we save when so many people are in an appalling situation everywhere," Fernando Calado, IOM's head of emergencies and post crisis division, said.
The migrants will be evacuated to Benghazi and taken by bus from there to Salloum in Egypt. Libyan rebels begged for more NATO air strikes on Thursday, saying they faced a massacre from government artillery barrages on Misrata, but Western allies squabbled over the air campaign. [ID:nLDE73D00A]
Aid organisations warn of a humanitarian disaster in Misrata, the lone rebel bastion in western Libya, where hundreds of civilians are said to have died in a six-week siege.
The ship is also bringing 400 tonnes of urgently needed humanitarian aid for residents of Misrata, including food and medical supplies for hospitals, according to the IOM. The goods were donated by groups including the Libyan Red Crescent.
At least 2,000 families who have fled fighting were being hosted by other families there or seeking makeshift shelter.
On Saturday, a ship docked in Misrata and delivered enough medical supplies to treat 300 war-wounded patients, the International Committee of the Red Cross said. [ID:nLDE73806F] (Reporting by Stephanie Nebehay; Editing by Giles Elgood)