Italy began a limited naval mission on Wednesday to help Libya's coast guard curb migrant flows, which have become a source of political friction ahead of national elections expected early next year.

An Italian patrol boat entered Libyan waters and headed towards the port of Tripoli within minutes of a vote in Italy's parliament authorizing the deployment. A second vessel was expected to join it in the coming days.

Italy announced the operation last week, saying it had been requested by Libya's UN-backed government. It initially hoped to send six ships into Libyan territorial waters, but the plans had to be scaled back following protests from Tripoli.

"[We will] provide logistical, technical and operational support for Libyan naval vessels, helping them and supporting them in shared and co-ordinated actions," Pinotti told parliament on Tuesday ahead of Wednesday's vote.

"There will be no harm done or slight given to Libyan sovereignty, because, if anything, our aim is to strengthen Libyan sovereignty," she added, stressing that Italy had no intention of imposing a blockade on Libya's coast.

In Tripoli, a poster of resistance hero Omar al-Mukhtar, who battled Italian rule in Libya in the 1920s, was hung over the capital's main square with the inscription "No to a return to colonization."

The lower house voted by 328 to 113 in favour of the mission. The upper house voted by 191 to 47.

LIBYA MIGRANTS

The number of arrivals is slightly down from the same time last year, but thousands are still coming to Italy. Here, African migrants en route to Italy are shown at a naval base in Tripoli on May 26 after being intercepted by the Libyan coast guard in the Mediterranean. (EPA)

After a surge in migrant arrivals on boats from Libya at the start of the year the numbers of newcomers has slowed in recent weeks and the interior ministry said on Wednesday that 95,215 people had reached Italy so far this year, down 2.7 per cent on the same period in 2016.

Some 2,230 migrants, most of them Africans fleeing poverty and violence back home, have died so far this year trying to make the sea crossing.

The votes came on a day in which the Italian coast guard seized a migrant rescue boat operated by a German aid group in the Mediterranean due to suspicions it had aided illegal immigration.

Video showed the "Iuventa," which is run by Jugend Rettet, arriving at the island of Lampedusa surrounded by several coast guard vessels after it was stopped at sea before dawn.

The Jugend Rettet group said on Twitter its crew was being interviewed by officials, but had received no information about an investigation into its activities.

The Human Rights Watch group said Italy's move may endanger migrants.

"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," HRW said in a statement.

Could become an election issue

The total number of migrants who have arrived in Italy over the past four years is some 600,000, putting Italy's network of reception centres under huge strain and causing increasing political tensions.

Italy is due to hold national elections by next May, with voting widely expected in early 2018, and the migrant issue is expected to top the political agenda. Rightist parties accuse the centre-left government of doing nothing to halt the influx.

"The [migrant boats] will not be being pushed back to the Libyan shore so we don't understand what we are going to be doing there," Giancarlo Giorgetti, deputy head of the opposition Northern League party, told reporters in parliament.

Italy hopes the Libyan coast guard can help prevent flimsy migrant boats from putting to sea and has been at the forefront of efforts to make the small force more effective, training its members and upgrading its fleet.

Rome has also put pressure on non-governmental organizations which have playing an increasingly important role in picking up migrants off the Libyan coast and bringing them to Italy.

The government has introduced a code of conduct for the NGOs and has demanded that armed police travel on their boats to help root out eventual people smugglers. Only three out of eight humanitarian groups operating in the southern Mediterranean agreed this week to the Italian terms.

Italy did not spell out the consequences for those that did not sign up, but on Wednesday, the Italian coast guard halted at sea a boat operated by German NGO Jugend Rettet, which had said 'No'. The vessel was searched and then escorted to port, while the identifications of crew members were checked.