Proactiva Open Arms, whose two ships sail back and forth in the Mediterranean to rescue migrants in distress, said it had decided to return a day after saving the 40-year-old woman from Cameroon.

In a statement explaining the decision to go back to Spain, the NGO said authorities in Italy had offered to take in the woman but not the two bodies.

Proactiva added it feared "for the protection of the surviving woman and her complete freedom to testify" on what had happened at sea if they left her in Italy, which has taken a hardline stance towards migrant arrivals.

Italy's far-right interior minister Matteo Salvini doubted their reasons, tweeting: "Could it be because they have something to hide?"

Earlier he denounced "lies and insults (that) confirm that we are doing the right thing: reducing departures and landings means reducing deaths, and reducing the profits of those who speculate on illegal immigration."

Proactiva accuses Libyan coastguards of having saved the rest of the migrants on board the dinghy but not the two women and the child, whom they say refused to board the rescue vessel and go back to Libya.

The NGO alleges that as a result, the coastguards left them and deflated the dinghy. Rescuers let air out of migrants boats to stop them from being re-used and this boat had been slashed with a knife.

- Rescue 'unfolded calmly -

Libyan coastguards denied Proactiva's accusations and said they rescued 165 migrants from a boat in the same area on Monday night, without leaving anyone on board.

They also pointed out a lack of resources, particularly for night operations.

"Coastguards save and protect lives... It is inconceivable for us to abandon people on the high seas when we have just gone to rescue them," the navy said in a statement.

Navy spokesman Ayoub Kacem added: "All these tragedies at sea are caused by the human traffickers who only want to make a profit, but also by the presence of these irresponsible NGOs in the zone."

A German TV journalist on board their Libya Star rescue boat told AFP the boat was "empty".

"There was absolutely nobody forgotten," Nadja Kriewald said.

"The evacuation unfolded calmly" and when the Libyans went down to remove the engine and pierce the boat with a knife, "the boat was empty".

The woman who survived has identified herself as Josepha and rescuers said she was found with hypothermia and was still in shock and unable to remember what happened.

Italy and Malta are cracking down on NGO boats operating in the Mediterranean, sharply reining in their rescue efforts as they accuse them of encouraging migrants to make the perilous crossing.

Proactiva Open Arms is currently the only humanitarian aid group operating in the central Mediterranean zone, which migrants try to cross to reach Italy or Malta.

The rescue ship's medical team said Josepha was in a stable condition but traumatised and needed medical and psychological treatment "as soon as possible".

The team also called for the quick transfer of the two bodies, as the Open Arms ship does not have refrigeration facilities to store bodies.