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," his colleague in Rome Federico Soda wrote in an email.
Soda said the organization had contacted the coast guard with the coordinates for the vessels, but "they do not have the assets to conduct these rescues right now."
"The weekend incident has tied up a lot of resources," he said, referring to the latest migrant tragedy Sunday when some 700 people were feared killed after a boat capsized off Libya.
"The coast guard will probably try to redirect commercial ships to the area," he added.
The announcement came as European Union interior and foreign ministers were due to meet to discuss the latest migrant tragedies.
Sunday's disaster followed a week in which two other migrant shipwrecks left an estimated 450 people dead, sparking calls for immediate action.
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.