Italy has seized a rescue ship operated by an NGO and dispatched a navy patrol boat to Libya as part of an Italian attempt to end the Mediterranean migrant and refugee crisis.

The twin moves came as figures revealed a surprise drop in July in the number of refugees and migrants arriving at southern Italian ports.

The Iuventa, a boat operated by German NGO Jugend Rettet, was impounded on the Italian island of Lampedusa on the orders of a prosecutor based in Trapani, Sicily, the state police force said.

“Inquiries begun in October 2016, and conducted with the use of sophisticated techniques and investigative technology, have produced circumstantial evidence of the motorboat Iuventa being used for activities facilitating illegal immigration,” a statement said.

Earlier, the Italian parliament authorised a naval mission in support of the Libyan coastguard’s fight against the people smugglers behind the surge in migrants and refugees attempting to cross the Mediterranean.

As part of the mission, approved by the Libyan authorities, the navy patrol boat entered Libya’s territorial waters on Wednesday afternoon, en route to Tripoli, the Italian navy said.

On board the Commandante Borsini was an advance team charged with coordinating planned joint actions with the Libyan coastguard.

The Italian defence minister, Roberta Pinotti, said Italy was providing technical support, not seeking to impose what she described as a “hostile” naval blockade designed to prevent the departure of migrant boats.

Italy’s prime minister, Paolo Gentiloni, said last week that the naval mission was being organised after a request from Fayez al-Sarraj, Libya’s UN-backed prime minister.

The number of migrant arrivals in Italy in July was down dramatically on the same month last year, suggesting efforts to train and better equip the Libyan coastguard could already be having an impact.

The interior ministry said 11,193 arrivals had been registered in July, compared with 23,552 in July 2016.

Arrivals for the first seven months of this year were 95,214, up 0.78% on the same period last year.

The naval mission is expected to involve a logistics ship and maintenance support for the Libyans, as well as the Commandante Borsini.

Rome had initially hoped to send six ships into Libyan waters but had to downsize the mission after objections from the fledgling administration in Tripoli.

About 600,000 people have arrived in Italy from Libya since the start of 2014, putting the country’s reception facilities under strain and the centre-left government under pressure with elections scheduled for next year.

Officials believe boats being sent back to Libyan ports will help deter people from paying traffickers for passage to Europe.

But the approach has been criticised by international rights groups who say people returned to troubled Libya face detention in squalid camps and abuse at the hands of smugglers.

“After years of saving lives at sea, Italy is preparing to help Libyan forces who are known to detain people in conditions that expose them to a real risk of torture, sexual violence, and forced labour,” said Judith Sunderland of Human Rights Watch.

The move to impound the Iuventa came as Italy began enforcing a controversial code of conduct for charity boats rescuing people in the Mediterranean.

Jugend Rettet was among six of the nine NGOs operating search-and-rescue activities in waters off Libya to reject the new rules.

Italian authorities say they are necessary to ensure the boats are not effectively encouraging migrants to embark on the perilous crossing.

The NGOs have particularly objected to a requirement to allow an Italian police official to travel on each boat and a ban on moving rescued people from one aid vessel to another while at sea, which they say could result in avoidable deaths.