The British coastguard raised the alarm on Wednesday about 59 African migrants it said were adrift without food or water on a leaking inflatable boat off the coast of Libya.
Those on board include 15 women, four of them pregnant, and seven children, the coastguard said in a statement. Their craft has no engine and is taking on water, it said.
The coastguard contacted a woman on the boat by satellite phone after being alerted by an Ethiopian man in Britain.
A British coastguard spokesman said he did not know the nationality of those on board, but believed they were sub-Saharan Africans who had apparently left North Africa two days ago trying to reach Europe.
They were dumped into the leaky craft by a commercial ship at night after having all their money taken from them, the coastguard said. They were told they were in Italian waters but in fact were inside the Libyan search and rescue region, some 64 nautical miles from the North African coast, it said.
"This is a truly awful episode in human trafficking ...," said Simon Rabett, a British rescue official. "We are endeavoring to work with all authorities to try and bring some form of humanitarian assistance to these desperate people."
The British coastguard said it had put out an emergency signal to merchant shipping in the area, and that a Tunisian commercial ship was now heading towards the craft.
The coastguard is also talking to Italian, Maltese, Libyan and Tunisian authorities, a spokesman said.