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 women, 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 (DfID) said it was "concerned" by the reports and seeking further information.
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."
"There must be unfettered and immediate humanitarian access in Libya to help those who are still trapped after weeks of being under siege.
"I pay tribute to the concerted efforts we have already seen from the international community to help protect the people in Libya and the work of organisations like the International Organisation for Migration who have helped so many to safety.
"Events today will only reinforce their determination to do all they can to protect people in Libya and step up the pressure on the Gaddafi regime."Reuse content