The dinghy was found 29 nautical miles off Zawyia, 40 kilometres (25 miles) west of Tripoli, Sea Watch said on Twitter, adding that several of the migrants, who included women and children, were given immediate medical attention.
Left-wing Italian humanitarian group Mediterranea, which also patrols the region in the Mare Jonio, said the Libyan coastguard “captured” another migrant boat earlier Wednesday near Tripoli.
“We had to watch powerless as Libyan militiamen intervened… violating all international conventions by pushing dozens of refugees towards the bombs and the torture they were trying to escape,” Mediterranea said in a statement.
The coastguard vessel sped past the Mare Jonio to reach “a boat that was nearly immobile and therefore in obvious difficulty”, it said.
When the Italian ship reached the dinghy, it was “empty… damaged and deflated, and as usual the engine was already gone”.
The German and Italian rescue ships were allowed to resume operations in the central Mediterranean at the beginning of June after the Italian authorities held them at port for two months citing “technical reasons”.
Warmer weather and better sailing conditions are expected to lead to a surge in attempts by migrants to reach Italy from Libya.