BERLIN — Germany on Monday called for the release of a boat captain detained in Italy after she defied authorities and docked a boat carrying 40 rescued migrants in a small Italian port, underscoring persistent tensions between European nations over migration.

The arrest of German captain Carola Rackete on Saturday came more than two weeks after she and other volunteers aboard the Sea-Watch 3 vessel rescued 53 migrants from a small dinghy in the Mediterranean in June.

She has quickly emerged as a symbol of resistance to what humanitarian groups have called inhumane policies to deter migration originating from North Africa. The tough rules, which severely restrict private groups from rescuing migrants from dangerous vessels in rough waters, have succeeded in partially slowing down the waves of people seeking refuge in Europe — but critics say it has come at a great human cost.

Rackete reignited the debate on Saturday when she pushed her 165-foot ship into the port of Lampedusa, leading Italy’s interior minister to brand her a “pirate” and call the docking “a criminal act, an act of war.”

Rackete is under house arrest in Italy and could face 10 years in prison.

German Foreign Minister Heiko Maas said on Twitter on Monday that “from our perspective only the release of #CarolaRackete can be the outcome of a trial under the rule of law.” He echoed prior remarks by German President Frank-Walter Steinmeier, who said Sunday that “someone who rescues people cannot be a criminal.”

Italy’s populist government has escalated its confrontation with private rescue groups in recent months, seeking to ban ships carrying migrants from its shores. Rackete had been told to return the migrants to war-torn Libya, after some were taken to Italy for emergency medical care.

But Rackete refused the order, arguing that laws of the sea required her to take the people to a safe port and that Tripoli did not qualify. After a 17-day standoff, she entered the Italian port, provoking a high-profile showdown with Italian authorities.

To her supporters across Europe, she is a hero, who — together with other volunteers of whom many are also German — has filled a vacuum European governments left as they scaled back their own rescue missions in the Mediterranean Sea.

The incident has exposed fissures between the governments of Italy and Germany, where pro-refugee sentiments remains stronger than in many other European Union member states. The German Protestant Church and other organizations have rallied behind sea rescue missions in the past, creating a broad momentum that has influenced the German government.

Stephan Weil, the Social Democratic leader of the German state of Lower Saxony, took aim at Italy’s right-wing interior minister Matteo Salvini, blaming him for an “intolerable escalation” with the arrest of the Sea-Watch 3 captain.

Liberal European politicians have criticized Italy’s moves to stop private rescue operations as a calculated effort to deter migrants from risking the journey by making it more deadly. This approach, they argue, comes at a high cost. About 2,300 people died trying to cross the Mediterranean Sea last year, according to the United Nations.

But Italy’s government has argued that private rescue missions are providing incentives for migrants to embark on risky journeys. Rome also has lashed out at other European nations for not opening their own ports to rescue vessels.

Italy has characterized Rackete’s actions as reckless.

As she was entering Lampedusa port, Rackete allegedly steered the vessel in the direction of a government boat that had sought to block her from entering. The Italian government condemned the maneuver as the act of a “pirate.”

But Gorden Isler, the voluntary chairman of Sea-Eye, another German group that conducts rescue missions in the Mediterranean, defended Rackete in a statement to The Washington Post. “Italy acted irresponsibly,” Isler said.

“It was their goal to wear the captain down, to provoke her and to force her into making a difficult decision,” Isler said.

Some German observers suspected the Italian government’s sharp rhetoric to be a political ploy. Migrant arrivals via the Mediterranean were today largely “symbolic,” as numbers have significantly declined, said Gerald Knaus, the founding chairman of the European Stability Initiative, a European think tank.

Repeated showdowns over rescue vessels funded by Germans seeking to enter Italian ports have mainly helped Italy’s far-right, which has dramatized the incidents to suggest the country “needs to guard itself against exploitation by hypocritical North Europeans,” Knaus said.

However, migrants arriving at Italian ports are typically later moved to countries with more welcoming attitudes toward refugees. Officials in Italy’s foreign ministry said the migrants aboard the Sea-Watch 3 vessel would be redistributed to Germany, France, Finland, Luxembourg and Portugal.

Knaus said the German government should make its de facto backing of private rescue vessels more explicit by announcing the country will consider giving asylum to all migrants rescued in the Mediterranean Sea.

In the end, people always end up in a few countries like Germany,” said Knaus, referring to migrants resettled after arriving in Italy.

In Germany, the large numbers of refugees who arrived in the country in recent years may in fact have boosted support for rescue missions in the Mediterranean Sea, Knaus said.

He added: “If you meet Gambians in Stuttgart or Senegalese in Bavaria who’ve been on some of those boats, it might actually motivate you to do more [to help others]."

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