Two charities running rescue missions in the Mediterranean Sea have said Italy has ignored requests to allow their ship to bring 356 migrants ashore, exposing Europe's latest failure to deal with African migration.

The Norwegian-flagged ship, Ocean Viking, already stranded at sea for 13 days awaiting port access, has been denied entry by Malta and its two requests to Italian authorities have gone unanswered, they said.

However, in a sign of a possible resolution, Portugal's government said late on Thursday it was ready to take up to 35 migrants and that France, Germany, Romania and Luxembourg were also willing to receive some of the people from the boat.