MISRATA, Libya (Reuters) - Libya put 142 illegal migrants on a flight back to Guinea on Wednesday with the help of the U.N. migration agency, officials said, as the North African country steps up deportations to ease severe overcrowding in its detention centers.

The number of people stuck in Libyan detention centers has risen dramatically this year after armed groups largely shut down the boat route to Italy from the smuggling hub of Sabratha.

The International Organization for Migration flew the Guinean migrants from two western cities to the city of Misrata, from where they boarded a plane for Conakry.

The deportations come in the wake of a CNN report on migrants being sold for slave labor that sparked an international outcry.

Migrant flows through Libya have surged since 2014. More than 600,000 people have crossed the central Mediterranean to Italy in the past three years, but departures from Libya's coast dropped sharply in July when armed groups in Sabratha began preventing boats from leaving.

After clashes in the western city in September, thousands of migrants held near the coast emerged and were transferred to detention centers under the nominal control of the U.N.-backed government in Tripoli.

Amnesty International said this month that up to 20,000 people are being held in detention centers and are subject to "torture, forced labor, extortion, and unlawful killings". Other human rights organizations have said similar things in recent months.

(Reporting by Ayman Salhi and Mostafa Salem; Writing by Ulf Laessing; Editing by Hugh Lawson)