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The United Nations called on Friday for in-depth investigations into the migrant boat sinking off Greece this week, believed to have left hundreds dead, and urgent action to prevent further tragedies.

On Wednesday, a fishing boat overloaded with migrants capsized and sank off the Peloponnese, killing at least 78 people. Around 104 people were found alive.

While the number of people on board at the time remains unclear, the UN pointed on Friday to various witness accounts suggesting it was somewhere between 400 and 750.

The UN agencies for refugees and migrants called in a joint statement for "urgent and decisive action to prevent further deaths at sea following the latest tragedy in the Mediterranean, the worst in several years".

They insisted states had an obligation to come together to address the dangerous gaps in search and rescue operations in the Mediterranean, pointing out that "the duty to rescue people in distress at sea without delay is a fundamental rule of international maritime law".

In particular, they rejected efforts to criminalise those who try to help in such situations, reiterating that "search and rescue at sea is a legal and humanitarian imperative".

The agencies also insisted that search and rescue operations should always be conducted in a manner aimed to prevent loss of life at sea, and said they welcomed that an investigation had been ordered in Greece into the circumstances that eventually led to the boat capsizing.

The UN rights office meanwhile called for "thorough" investigations into the "horrific tragedy".

Spokesman Jeremy Laurence told reporters in Geneva there was a need to investigate "people smugglers and human traffickers and ensure they are brought to justice", and that more broadly "there are a lot of questions that need to be asked".

Federico Soda, head of the International Organization for Migration's emergency department, said the tragedy once again showed the approach to migrant crossings in the Mediterranean needed to change.

"It is clear, that the current approach to the Mediterranean is unworkable," he said in the statement.

"Year after year, it continues to be the most dangerous migration route in the world, with the highest fatality rate."

UNHCR deputy chief Gillian Triggs also stressed the obligation to "put safety and solidarity at the heart of its action in the Mediterranean".

"In view of the increased movements of refugees and migrants in the Mediterranean, collective efforts, including greater coordination between all Mediterranean States, solidarity and responsibility sharing… are essential to save lives," she said in the statement.