A UK-funded aid ship has been caught in Misrata harbour during a bombing raid by Muammar Gaddafi's forces that is believed to have killed four people.
The Red Star One docked at the besieged Libyan port to deliver 180 tons of humanitarian supplies and evacuate 1,000 stranded migrant workers and injured civilians.
The Libyan Red Crescent said the attack killed four migrants - a woman, a man and two children who were in a camp nearby. The ship, operated by the International Organisation for Migration, had spent three nights outside Misrata waiting to enter because the sea approaches have been mined.
The Department for International Development said it is "concerned" by the reports and seeking further information.
The ship was said to have left the port for the opposition stronghold of Benghazi on Wednesday evening.
International Development Secretary Andrew Mitchell paid tribute to the "tremendous bravery" of the crews of ships trying to reach Misrata.
"The ship that went in today was funded by the British taxpayer," he told Channel 4 News. "It took out today, in the very difficult circumstances that you have described, about 800 of those poor and dispossessed migrant workers and some of the casualties."
Mr Mitchell went on: "While Nato is making progress it is extremely difficult to defend the people who are caught there in the port. That is particularly because the artillery that is being used against them is dug in there in Misrata in civilian accommodation. Because of the great care that Nato is taking not to kill civilians it is simply impossible for them to target these guns."
Shadow international development secretary Harriet Harman said: "These reports from Misrata are horrifying. Innocent civilians continue to suffer because of the actions of the Gaddafi regime.
"Despite the short time they had, the brave efforts of humanitarian aid workers and the crew on board this British-funded ship helped rescue hundreds of stranded people, but they should have had the chance to do much more. The days of delays they have faced have been completely unacceptable."