A charity rescue ship in the Mediterranean said Monday it was racing to help a migrant boat in distress, but feared doing so might break the Italian government's new law on life-saving missions.

"We are heading towards the boat in danger, to assist," Doctors Without Borders press officer Maurizio Debanne told AFP, after the Geo Barents ship was told some 45 to 50 people were stranded at sea.

But he said it fears falling foul of a new decree signed into law Monday, aimed at restricting the numbers being brought to shore by limiting the number of rescues charities can make at any one time.

Rescues must be signed off on by Rome, which can order charities not to assist people, Debanne said.

Doctors Without Borders (MSF) published a series of tweets earlier Monday making it clear it was only aiding boats after being requested to by Italian authorities.

It also said Rome had asked the Geo Barents in one case not to assist, because the situation was "being managed by Libya".

The ship obeyed, and began heading instead to the port of Taranto, with 85 rescued people on board.

It then received an alert from Alarm Phone, a hotline used by migrants in distress, about a boat in trouble on their route.

"We immediately requested permission from the Italian authorities to intervene, but received no reply," Debanne said.

"If we find the boat, and perform the rescue without permission, Italy could theoretically say we have broken the new law.

"But under international law and maritime conventions, we are obliged to assist those in danger."

The Geo Barents was not going off course to look for migrants, he insisted, saying those in trouble were on its route to port.

Prime Minister Giorgia Meloni's hard-right government took office in October, vowing to stop migrant landings in Italy, which reached more than 105,000 in 2022, according to the interior ministry.

She claims migrants and people traffickers are encouraged by the charities which rescue people attempting the perilous crossing from North Africa.

However, the charities say they always follow international and national laws regarding saving lives at sea, and slam the new rules as putting lives at risk.

Italian asylum lawyer Fulvio Vassallo Paleologo told AFP the new law introduced a system of "selective rescues" which "violates the primary value of safeguarding human life" enshrined in international law.

Most of the migrants who arrived in 2022 were rescued and brought to shore by Italian navy or coast guard vessels, not charity ships.

At least 20,218 people have died or gone missing on the crossing -- the world's deadliest -- since 2014, according to the International Organization for Migration (IOM).