Seventy migrants headed for Europe across the Mediterranean from Libya have gone missing, the independent support group Alarm Phone said Saturday.

"A boat with 70 people on board is missing for 4 days! They departed from Khoms, #Libya, & called AlarmPhone several times," it tweeted.

"When we lost contact, they were in Malta SAR (search and rescue) zone, 11 miles (18 kilometres) to Italian waters, but there is no record of their rescue or arrival. Authorities are silent."

Alarm Phone criticised authorities in Malta and Italy.

"Why did Maltese and Italian authorities refuse first to rescue, then to provide information on their fate? Were the 70 people left to die? Were they secretly pushed back to #Libya? We demand answers!" it said.

Tens of thousands of people seek to cross the central Mediterranean each year, normally setting off from Libya and aiming for the Italian coast, but it is often a deadly journey.

At least 1,369 people have died on the route so far this year, the International Organization for Migration (IOM) said in early September.