European rescue agencies and coastguards clashed on Monday, sowing confusion over the fate of 85 migrants feared to have drowned after setting off from Libya.
Germany's Sea-Watch International reported that four rubber boats carrying 258 people had run into difficulty in the Mediterranean.
It said it lost contact with one boat carrying 85 people somewhere between the territorial waters of Libya and Malta.
The EU’s Frontex coastguard on Sunday said it was “very worried” about the passengers onboard.
But the agency on Monday said that all four boasts had been accounted for and there had been no shipwreck in the area.
"Two arrived in Sicily and two remain in the Maltese search and rescue area," Frontex said.
It said a search and rescue plane was continuing to search to "make sure that no other vessels in distress are in the area".
But Sea-Watch International stood by its report.
"For days, at least three boats with more than 150 people are in distress in the Mediterranean Sea, a fourth sunk already," the German group said on Twitter on Monday.
Carlotta Sami, a spokeswoman for the UN refugee agency, said it appeared that the fourth dinghy had capsized.
"We are very worried," Ms Sami said.
Frontex on Monday said there was a "deflated rubber boat in the Libyan search and rescue area", but that this was left from an earlier rescue mission carried out by the Libyan coastguard.
Italian authorities "later confirmed that this boat correlated with an old incident, where the migrants were rescued and the rubber boat was left adrift empty", it said.
The Italian coastguard on Monday said its Libyan equivalent had "in recent days" conducted its own search and rescue mission.
The Libyans had "subsequently left the boat drifting empty, rescuing the migrants who were on board", it said.
Two German groups who monitor dangerous migrant crossings first reported the vessels had run into difficulty.
Sea-Watch tracked the location of the boats, including one vessel marked "unknown, GPS contact lost", on its official Twitter account.
The United4Rescue monitor said it was receiving the same reports and feared for the lives of "dozens".
More than 500 migrants set out from Libya last week, according to the International Organisation for Migration. Their fates have been complicated by the pandemic threat.
Italy has long been a primary point of entry to Europe for migrants making the dangerous journey across the Mediterranean.
But the country shut down its ports amid the coronavirus pandemic, and said it would quarantine any illegal migrants.
Malta last week suspended migrant rescue operations because of coronavirus fears, even as it brought ashore 70 people who had entered its territorial waters after drifting at sea for five days.
About 280 migrants were picked up by the Libyan coastguard after three days at sea, but then denied entry to the Libyan capital, Tripoli, after the government declared its seaports unsafe.
The migrants organisation said the latest reports on the possible capsize were "very worrying" but difficult to verify.
"In the absence of boats in the area, it is very difficult at the moment to confirm that there has been a shipwreck, or the number of victims involved," said Flavio Di Giacomo, the group's spokesman for Italy.
"And unfortunately, from experience, we also think it is likely that there have been shipwrecks of which we are not aware."
The central Mediterranean route remains one of the most dangerous journeys for migrants trying to reach Europe.
At least 134 people are known to have died on the route this year, data from the IOM’s Missing Migrants project shows.