IOM's Rome office has received a call for help from one of three boats floating near each other in international waters, spokesman Joel Millman said.
"The caller said that there are over 300 people on his boat and it is already sinking (and) he has already reported fatalities, 20 at least," he wrote in an email, adding that no further details were yet available.
The announcement came as European Union interior and foreign ministers were due to meet to discuss the latest migrant tragedy Sunday when some 700 people were feared killed after a boat capsized off Libya.
Some 11,000 migrants have been rescued since the middle of last week alone and current trends suggest last year's total of 170,000 landing in Italy is likely to be exceeded in 2015.