A commercial ship rescued 35 Europe-bound migrants off Libya's Mediterranean coast and returned them to the capital, Tripoli, the UN migration agency said.

The International Organization for Migration posted on Twitter that the migrants, intercepted on Thursday, were given medical assistance and relief items upon disembarkation.

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"Saving lives at sea is a moral and legal obligation. It is, however, unacceptable that migrants continue to be returned to an unsafe port," said the IOM.

Libya, which descended into chaos following the 2011 uprising that toppled and killed longtime ruler Muammar Gaddafi, has emerged as a major transit point for Africans and Arabs fleeing war and poverty in their home countries and hoping to travel to Europe.

Most migrants make the perilous journey in ill-equipped and unsafe rubber boats. As of last October, roughly 19,000 people had drowned or disappeared on the sea route since 2014, according to IOM.

Last week, a rubber dinghy packed with 91 migrants set out from Libyan shores for Europe; it went missing in international waters in the Mediterranean Sea.

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In recent years, the European Union has partnered with the coastguard and other forces in Libya to stop the flow of migrants.

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

The latest developments come amid criticism of the EU's lack of rescue missions in the Mediterranean Sea.

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