Egyptian police moved some 350 Eritrean migrants to Cairo from detention on the Red Sea coast on Friday in preparation to fly them home, police sources said.

The UN refugee agency UNHCR has objected to the mass deportations, which could violate the prohibition on sending people home who have a well-grounded fear of persecution.

Activists close to the migrant community say they think the Egyptians have deported at least 810 Eritrean asylum seekers since June 11, out of about 1,600 Eritreans who were in Egyptian detention. The government has not given any figure of its own.

The police sources in the Red Sea said the latest would leave for the Eritrean capital from Cairo airport overnight. Cairo airport sources said they were not aware of any preparations for handling the migrants.

The deportations are the largest forced returns of asylum seekers from Egypt in decades, and could mark a shift in Egypt's policy toward tens of thousands of largely African migrants on its territory, activists say.

Some of the Eritreans are believed to be economic migrants who aim to slip across the border into Israel to seek work.

Amnesty International says thousands of migrants try to cross into Israel from Egypt each year, with numbers rising since 2007. But Eritreans arriving in Egypt in recent months include Pentecostal Christians fleeing religious persecution and others trying to avoid military conscription, activists say.

The Egyptian government has said it fulfils its commitments under international law on refugees but UNHCR said on Thursday that the Egyptian authorities were obstructing access to the asylum seekers, who could face torture in Eritrea.

A UNHCR spokeswoman said she had no information about the latest preparations. In some cases the Egyptians have moved Eritreans from one site to another and it was not possible to find out later if they had left Egypt, she added.