As many as 5,000 migrants have drowned attempting to cross the Mediterranean Sea to Europe this year, UN agencies have said, a new high for the crisis.

Two overcrowded boats capsized off the coast of Italy with at least 90 people missing and feared dead, Joel Millman, a spokesman for the International Organisation for Migration (IOM) said on Friday.

The nearly 100 missing from those two incidents have brought the death toll to a record level, he said,

Sixty-three survivors from the capsized boats said there had originally been between 120 and 140 people onboard, Mr Millman told a Geneva briefing.

"Those two incidents together appear to be the numbers that would bring this year's total to 5,000 [deaths], which is a new high in this crisis," he said.

UNHCR spokesman William Spindler said the was "the worst annual death toll ever seen".