The number of people who have died trying to cross the Mediterranean nearly doubled in the first half of 2021, the International Organisation for Migration said Wednesday.

At least 1,146 people died attempting to reach Europe by sea in the first six months of 2021, the IOM said in a statement.

It said 513 migrants are known to have drowned in the same period last year, compared with 674 in the first six months of 2019.

"Civilian search and rescue organisations continued to face significant obstacles to their operations, with the majority of their boats blocked at European ports due to administrative seizures and ongoing criminal and administrative proceedings against crew members,” the statement said.

The agency also noted that the increased deaths come as interceptions of migrant boats launched from Libya are on the rise.

Those who have been returned to Libya are subjected to arbitrary detention, extortion, disappearances, and torture, the IOM said.

It also warned that figures for migrants braving the western Mediterranean route may be severely under-estimated.

"Hundreds of cases of invisible shipwrecks have been reported by NGOs in direct contact with those on board or with their families,” it said.

The body called for "urgent and proactive steps to reduce loss of life on maritime migration routes to Europe,” quoting IOM Director General Antonio Vitorino.

Italy and the European Union have for years been financing, training and providing aid to Libya’s coastguard to stop smugglers from taking migrants and refugees in flimsy boats across the Mediterranean to Europe.

But the coastguard has faced numerous accusations of appalling mistreatment of asylum seekers, and charities and human rights groups have severely criticised the arrangement.

In Wednesday’s statement, Vitorino urged stepped-up search-and-rescue efforts, as well as "establishing predictable disembarkation mechanisms and ensuring access to safe and legal migration pathways”.