A European rights watchdog rapped NATO on Thursday for failing to help a migrant boat during the Libyan war as aid groups vowed to sue the French military over an incident that left 63 people dead.

A small boat left Tripoli in late March 2011 carrying 72 Africans, including 50 men, 20 women and two babies, but only nine survived after two weeks adrift in the Mediterranean, the Council of Europe said.

"Many opportunities for saving the lives of the persons on board the boat were lost," a council committee said in a report denouncing a "catalogue of failures" by Libyan, Italian, Maltese and NATO authorities.

Several Western warships were in the Mediterranean at the time enforcing a NATO-led arms embargo, but no one including fishing vessels in the area responded to distress calls, the report said.

"NATO failed to react to the distress calls, even though there were military vessels under its control in the boat's vicinity when the distress call was sent," said the report presented by Dutch socialist senator Tineke Strik.

It said a Spanish frigate, the Mendez Nunez, was 11 miles away from the drifting boat but Spain disputes this distance.

A helicopter dropped biscuits and water to the migrants at one point but never returned, and a large military ship "came into close contact with the boat, but ignored obvious distress signals," the council report said.

Bilal Yacoub Idris, a 30-year-old survivor from Ethiopia, recalled how the migrants were tightly packed in the dinghy after paying smugglers to flee the Libyan conflict towards Europe.

"It was completely overcrowded. Everyone was sitting on everybody else. I had someone sitting on top of me, and this person had someone sitting on top of him," the report quoted him as saying.

Stephane Maugendre, a lawyer and president of the French Immigrant Worker Support and Information Group (GISTI), announced that it would file a lawsuit in Paris next month in the name of survivors.

"We will file a complaint against the French armed forces for failure to assist people in danger," Maugendre said at a press conference in Brussels, adding that similar lawsuits would be filed in other European nations.

NATO spokeswoman Oana Lungescu said the alliance received a general notice from Italian authorities of a small boat probably in difficulty on March 27 last year and that the message was passed on to all of its ships.

There is no record of any NATO ship or aircraft having seen or making contact with the boat, she said, adding: "NATO conducted other rescue operations in the area at that time, and rescued hundreds of people."

NATO ships and aircraft helped rescue more than 600 people in the seven-month-long Libyan operation during the uprising which overthrew dictator Moamer Kadhafi, she said.

On March 26 last year, the 72 migrants sailed towards Europe but their boat quickly ran out of fuel and began drifting, according to the report.

The passengers, lacking food or water, managed to call an Eritrean priest based in Rome who alerted the Italian maritime authorities. Hours later, a helicopted flew over the boat and delivered water and biscuits.

It was the last time they would get any help, even though they saw two fishing vessels, a frigate believed to be from Spain and a warship believed to be Italian.

"Some people were hallucinating and speaking incoherently, perhaps because of drinking seawater," the report said. "Many could not sleep, and one young woman threw herself into the sea in a panic attack."

Another survivor is quoted as saying: "Every day, there were more and more people who would die."

Many people started to die on the fifth or sixth day at sea, including the children. Around half were dead by the 10th day and were thrown overboard due to the stench.

"While we were talking to one another, four of us just died, four of the people in that group, talking, just passed away," said Bilal, the Ethiopian.

On the 15th or 16th day, only 11 people were still standing.

"We were just waiting for our own time or turn to die," said Ghirma Halefom, an Eritrean.

On April 10, the boat landed on the Libyan coast. Ten survivors were arrested and put in prison. One of them died while the rest eventually managed to escape Libya.