The International Organization for Migration said Monday it had received a distress call from a sinking boat in the Mediterranean carrying more than 300 people, with at least 20 people reported dead.
IOM's Rome office said it had received a call for help from one of three boats floating near each other in international waters.
"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," IOM spokesman in Rome Federico Soda wrote in an email.
His colleague Flavio Di Giacomo however warned that so far IOM only had the word of the caller to go by.
"For now, this is simply a call for help... It's too soon to talk about a shipwreck," he told RaiNews24.
He said migrants often make distress calls even when they are not in a desperate situation to ensure they get picked up.
IOM said it had contacted the coast guard with the coordinates for the vessels.
But Soda warned "they do not have the assets to conduct these rescues right now."
"The weekend incident has tied up a lot of resources," he said, adding that the coast guard would "probably try to redirect commercial ships to the area."
Italian and Maltese navy boats were still scouring waters off Libya for the victims of a shipwreck Sunday feared to have cost the lives of hundreds of people.
But the outlook appeared grim, with only 28 survivors rescued so far, along with 24 bodies picked up by the Malta coast guard.
One survivor told Italian authorities that there were as many as 950 people on board and that some of them had been locked below deck by the smugglers.
An IOM team has for the first time manage to gain access to the Malta port, but had so far not been able to speak to any of the survivors, spokesman Joel Millman said.
Sunday's tragedy came after a week in which two other migrant shipwrecks left an estimated 450 people dead.
If the worst fears about Sunday's tragedy are confirmed, it would take the death toll since the start of 2015 to more than 1,600, sparking calls for immediate action.
European Union interior and foreign ministers met Monday to discuss the latest migrant tragedies.
Every day, between 500 and 1,000 migrants are picked up by the Italian coast guard in the Mediterranean, and some 11,000 migrants have been rescued since the middle of last week alone.
The current trends suggest last year's total of 170,000 landing in Italy is likely to be exceeded in 2015.
According to the UN refugee agency, some 35,000 migrants have already arrived in southern Europe by boat so far this year.