"Some 340 refugees and migrants have been returned today to Tripoli by Libyan Coast Guard," UNHCR tweeted, Xinhua reported.
"UNHCR and IRC (International Rescue Committee) provided urgent medical and humanitarian assistance to all survivors before being taken to detention," UNHCR said.
Libya has become a preferred point of departure for thousands of immigrants who attempt to cross the Mediterranean Sea to reach European shores.
According to the International Organization for Migration, more than 6,000 illegal migrants have been rescued off the Libyan coast so far this year, including women and children.
Rescued migrants end up in overcrowded reception centers across Libya, despite repeated international calls to close those centers.
ZZ/XNA