BRUSSELS (AP) — The United Nations refugee agency urgently appealed to European governments Tuesday to let two migrant rescue ships disembark more than 500 passengers who remain stranded at sea as countries bicker over who should take responsibility for them.

The people rescued while attempting to cross the Mediterranean Sea from North Africa are on ships chartered by humanitarian aid groups, which the Italian government has banned from its territory. The archipelago nation of Malta has refused to let the ships into the country's ports.

It's unclear where they might find safe harbor, even though the Italian island of Lampedusa appears closest. About 150 of the rescued passengers have been on the Spanish-flagged charity ship, the Open Arms, since they were plucked from the Mediterranean 13 days ago.

"This is a race against time," Vincent Cochetel, UNHCR special envoy for the Central Mediterranean, said in a statement. "Storms are coming, and conditions are only going to get worse."

While the number of migrants reaching Europe by sea has dropped substantially so far this year, UNHCR says nearly 600 people have died or gone missing in waters between Libya, Italy and Malta in 2019.

The agency said many of the people onboard the ships "are reportedly survivors of appalling abuses in Libya." Cochetel said the ships "must be immediately allowed to dock" and their passengers "allowed to receive much-needed humanitarian aid."

"To leave people who have fled war and violence in Libya on the high seas in this weather would be to inflict suffering upon suffering," the envoy said.

The captain of the Open Arms, Marc Reig, sent a letter Monday to the Spanish Embassy in Malta asking Madrid to grant asylum to 31 minors on his ship. A senior Spanish official said Tuesday that Reig's request carries no legal weight because the captain doesn't have authority to seek protection for the minors.

The limbo of the Open Arms and a Norwegian-flagged ship operated by Doctors Without Borders and sea rescue group SOS Mediterranee is the latest in a string of standoffs that kept Europe-bound migrants at sea in miserable conditions.

Southern nations that have been the main arrival points since 2015 - notably Italy, but also Malta and Greece - have complained of feeling abandoned by their European Union partners to cope with the influx.

Italy's hard-line interior minister, Matteo Salvini, reiterated Tuesday his intent to ensure that the ships don't enter Italian ports.

Differences among EU member nations over how to manage mass migration have sparked a political crisis in Europe, while attempts to reform the bloc's asylum system have failed. The issue has been a vote-winner for far-right and populist parties.

The EU's executive commission said it has urged member countries to take action to resolve the status of the recently rescued passengers and stands ready to offer national governments support but cannot act alone.

"There's nothing more we can do," a European Commission spokeswoman said Tuesday.

Doctors Without Borders (MSF) said late Monday in a tweet that "a critical rescue" had brought 105 more people onto the Ocean Viking, raising the number of migrants aboard the vessel to 356.

Giada Zampano in Rome and Barry Hatton in Lisbon contributed to this report.