(CNN)A ship carrying more than 600 rescued migrants, including 123 unaccompanied minors and seven pregnant women, remained stranded in the Mediterranean Sea on Monday after Italy's new populist government refused permission for it to dock the day before in a move described by rescuers as unprecedented.

Matteo Salvini, Italy's hardline interior minister and leader of the anti-immigration League party, called on the country to "close the ports" and pledged Sunday to "STOP the filthy business of illegal immigration."

The search-and-rescue ship Aquarius is operated by Doctors Without Borders (MSF) and the SOS Méditerranée organization. From Saturday night into Sunday morning, the Aquarius took on 629 people in six separate operations, MSF and SOS said on Twitter.

The boat has been stranded between Malta and the Italian island of Sicily since Sunday, when the Italian Maritime Rescue Coordination Center told Aquarius to hold its current position, according to MSF.

The ship is equipped for only around 550 people, according to Aloys Vimard, project coordinator on the Aquarius. Speaking to Spanish public radio broadcaster RNE Monday, Vimard said that on-board food supplies would last only one more day.

"MSF is concerned that again politics are being placed above people's lives," Vimard told CNN. "The priority must be the importance of the well-being and safety of the people on board."

A number of passengers are in need of medical treatment, MSF tweeted, adding that 15 have serious chemical burns and several others have hypothermia. Dozens of people had spent almost a full day in the water by the time the Aquarius reached them, Laura Garel, communication officer with SOS Méditerranée France, told CNN on Monday.

None of those rescued are aware of the "ongoing diplomatic standoff," MSF tweeted Monday, calling for "a swift resolution and a designated port of safety."

In footage posted to Twitter by Euronews and NBC journalist Anelise Borges, women were sitting and lying on the floor in an enclosed area below deck, one fanning another to combat the heat. "It's unbearably hot where they are," Borges tweeted.

There are no showers or sinks for migrants on the Aquarius and rescue workers use a disinfectant spray on people's hands before they eat. Drinking water supplies are unlimited as the ship has its own water purification system, but the passengers survive mostly on energy biscuits, prepackaged warm rice, tea and bread.

Women and children sleep below deck, watched over by a midwife, while the men sleep on the open deck, covered only with tarpaulin.

Tensions in the Mediterranean are likely to rise as more rescue ships head for the Italian coast. According to Flavio di Giacomo, spokesman for the International Organization for Migration, an additional 790 migrants were rescued Sunday by the Italian Coast Guard.
"I don't think the Italian government can deny a port of safety to the Italian Coast Guard, but they can to a foreign-flagged ship like the Aquarius," he told CNN Monday. The Aquarius flies the flag of Gibraltar.

Also on Monday, Salvini signaled his intention to prevent a second foreign-flagged search-and-rescue vessel, Sea Watch 3, operated by a group of German volunteers and currently positioned off the coast of Libya, from docking.

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