The International Organization for Migration tweeted that most of the 301 migrants, who were on three boats and intercepted Saturday, were taken to detention centers in Libya. It said that some managed to escape at the disembarkation point, as the boats were brought back to shore.

"It is unacceptable for this to continue despite repeated calls to put an end to the return of vulnerable people to detention and abuse," said Safa Msehli, a spokeswoman for the IOM.

Libya, which descended into chaos following the 2011 uprising that toppled and killed longtime dictator Moammar Gadhafi, has emerged as a major transit point for African and Arab migrants fleeing war and poverty to Europe.

Most migrants make the perilous journey in ill-equipped and unsafe rubber boats. The IOM said earlier this month that its estimated death toll among migrants who tried to cross the Mediterranean past the "grim milestone" of 20,000 deaths since 2014.

In recent years, the European Union has partnered with the coast guard and other Libyan forces to stop the flow of migrants.

Rights groups say those efforts have left migrants at the mercy of brutal armed groups or confined in squalid and overcrowded detention centers that lack adequate food and water.

The EU agreed earlier this year to end an anti-migrant smuggler operation involving only surveillance aircraft and instead deploy military ships to concentrate on upholding a widely flouted U.N. arms embargo that's considered key to winding down Libya's relentless war.