Malta stopped a small aircraft used to search for migrant boats in the Mediterranean from operating out of the country, the German maritime rescue NGO Sea Watch said.
A Maltese government spokesman confirmed the aircraft was grounded and a full statement was expected later.
The move appeared to be part of efforts by both Malta and Italy to make it near impossible for non-governmental organisations (NGOs) to use them to help rescue migrants adrift off Libya.
“From now on no search flights in the search area north of the Libyan coast are possible anymore,” Sea Watch said.
It said the Moonbird aircraft, operated by Sea Watch and the Swiss Humanitarian Pilots Initiative, was involved in the rescue of about 20,000 people.
“More than 1,000 would have drowned if the Moonbird crew had not spotted their sinking boats,” it said.
Last week Malta closed its harbours to NGO ships after allowing a vessel operated by the German charity Mission Lifeline to dock and disembark 133 migrants.
Italy and Malta criticised the ship for defying Italian coastguard orders and picking up migrants rather than leave the task to the Libyan Coastguard.
The ship’s captain was taken to court in Malta on Monday and accused of registration irregularities. The Lifeline was detained pending further investigations.
Earlier Malta followed Italy in banning the rescue ship Aquarius, operated by Doctors Without Borders, forcing it to undertake a long trip to Spain to disembark migrants there.
Another NGO rescue ship, the Sea Watch III, was blocked from leaving Valletta Harbour on Sunday, for undisclosed reasons.
Malta, like Italy, insists NGO ships must not obstruct national coastguards from rescuing migrants.