Two charities operating rescue missions in the Mediterranean Sea have said that Italy has ignored requests to allow their ship to bring 356 people ashore.

The Norwegian-flagged vessel, 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, the charities said.

The ship is carrying mostly Sudanese migrants, rescued from the sea in four separate missions.

They include more than 100 minors, around 90 of them unaccompanied, Médecins Sans Frontières (MSF) said. Three children are under the age of five.

The plight of the Ocean Viking, run by MSF and another French charity, SOS Méditerranée, threatens to create another migration stand off with Italy if it chooses to head there, after weeks of controversy involving another charity-run rescue ship.

Around 100 people were stranded off Italy for almost three weeks on the Open Arms vessel until an Italian prosecutor intervened and ordered they be brought ashore against Interior Minister Matteo Salvini's wishes. Five of Italy's European partners will take the migrants in.

"This is shameful," said Jay Berger, project coordinator for MSF, speaking by satellite phone from the Ocean Viking, in international waters between Malta and the southern Italian island of Linosa.

"Leaving migrants on rescue boats for weeks until the crisis becomes an emergency is becoming the new norm," he said. "We're not trying to force our way into Italian or Maltese waters. We're waiting for a solution but it is taking too long."

The ship requested port access to Italian authorities on 9 and 12 August, the charities said.

An Italian Transport Ministry spokesman declined to comment. Italy's Foreign Ministry did not respond immediately to an email seeking comment.

Rome has banned entry to private rescue ships, which operate off Libya in international waters close to Italy. Mr Salvini calls them "taxis" for people smugglers and says Italy should not be "Europe's refugee camp".

Supplies running down on ship

Aboard the Ocean Viking, supplies are running down and the crew has begun to ration showers to conserve water, said Frédéric Penard, SOS Mediterranee's director of operations.

"A rescue ship is like an ambulance. People should be transported but not living on it," Mr Berger said, adding that there was no certainty those on board could endure the wait Open Arms experienced, which would mean another week at sea.

No port except for Tripoli has welcomed the ship, the charities say. They regard the Libyan port as unsafe and fear the migrants would be thrown into detention centres and suffer human rights abuses if returned to the country.

Luca Pigozzi, a doctor on the vessel, said that while most migrants were in a stable physical condition, he feared the impact of psychological damage caused by violence suffered when fleeing their home country.

"The situation onboard is becoming more tense," he said.

Libya is a hub for migrants and refugees, many of whom try to reach Europe in unseaworthy boats, fleeing violence and poverty in Africa for a better life in Europe.

So far this year, more than 600 people have died trying to make the crossing.