Italy yesterday doubled down on its new tough stance against migrants, calling for a German NGO's rescue boat to be impounded and its crew arrested as the EU prepares for a tense weekend mini-summit on the issue dividing the bloc.

Just three weeks in power, Italy's new populist government is digging its heels in on campaign promises to stop the influx of migrants, threatening to seize rescue ships or barring them from its ports.

This time, it has set its sights on the German NGO, Mission Lifeline.

"The illegal boat Lifeline is now in Maltese waters with its cargo of 239 migrants. For the safety of its crew and the passengers, we've asked Malta to open its ports," Interior Minister Matteo Salvini wrote on Twitter.

"Clearly, the boat should immediately be impounded and its crew arrested," said Salvini, who is also Italy's deputy prime minister.

A source close to the Maltese government said that Valetta had not yet received any official request from Rome, nor had Lifeline itself sought any permission to land.

Salvini has repeatedly accused NGOs of being complicit with human smugglers operating in Libya.

"Italian ports are no longer at the disposal of traffickers. Open the Maltese ports! Open the French ports," he said yesterday, speaking ahead a second round of local elections on Sunday in Sienna.

On Thursday, Italy's infrastructure minister Danilo Toninelli had said that two of Mission Lifeline's ships -- the Lifeline and the Seefuchs -- would be "seized by the Italian government and directed into our ports" to launch an investigation into their legal status.

Meanwhile, the UN said Thursday more than 200 migrants are reported to have drowned off Libya in two days, bringing to at least 1,000 the number to die this year on the main migratory route linking Africa and Europe.

The UN High Commissioner for Refugees said in a statement it was "shocked" by the deaths and called for urgent action to reduce the continuing loss of life.

According to the UNHCR, around 220 migrants drowned on Tuesday and Wednesday in three separate tragedies.