Hundreds of migrants intercepted off the coast of Libya
Photos posted by the IOM Twitter feed showed workers distributing blankets to migrants after they arrived onshore, but the agency also voiced its objection to the treatment of migrants inside the war-torn country.
"We maintain that Libya is not a safe port," it wrote.
It said that 310 migrants were brought back to shore on Saturday night, and an additional 173 were returned on Sunday.
They are among the 4,500 people who have been brought back to Libya this year after attempting the dangerous Mediterranean Sea crossing to Europe, the International Organization for Migration said in a tweet Sunday.
In the years since the 2011 NATO-backed uprising that ousted and killed longtime dictator Moammar Gadhafi, Libya has emerged as the dominant transit point for migrants fleeing war and poverty in Africa and the Middle East.
Smugglers often pack desperate families into ill-equipped rubber boats that stall and founder on the perilous central Mediterranean route.
In recent years, the European Union has partnered with Libya's coast guard and other local groups to stem such dangerous sea crossings.
Rights groups say those policies leave migrants at the mercy of armed groups or confined in squalid detention centers rife with abuses.
On January 19, a boat carrying migrants bound for Europe capsized in the Mediterranean off the coast of Libya, and at least 43 people drowned.
The tragedy marked the first maritime disaster in 2021 involving migrants seeking better lives in Europe. The IOM had cited survivors as saying that the dead were all men from West African nations.