U.N. aid chief Martin Griffiths said on Thursday he was hopeful that the first shipment of grain from a Ukrainian Black Sea port could take place as early as Friday, but "crucial" details for the safe passage of vessels were still being worked out.

The Lloyd's of London [SOLYD.UL] facility will provide up to $50 million of cover in marine cargo and war insurance, Lloyd's, Ascot and Marsh said in a statement.

The cover would "add essential protections to the deal brokered by the UN last week and represents the latest support from Lloyd's and the insurance industry to help the international community respond to the conflict", said Patrick Tiernan, chief of markets at Lloyd's.

(Reporting by Carolyn Cohn; editing by Saikat Chatterjee and Jason Neely)