Seven Eritrean asylum seekers have been transferred to an Israeli prison indefinitely after refusing to be deported to Rwanda, Israeli human rights groups reported.

The asylum seekers are the first to be placed in indefinite detention since the Israeli government announced it would force tens of thousands of African migrants to choose between deportation to a third country or incarceration for an unspecified length of time.

"This is the first step in a what is a globally unprecedented deportation operation, a move tainted by racism and complete disregard for the life and dignity of asylum seekers," the Hotline for Refugees and Migrants and ASSAF, two Tel Aviv-based refugee rights groups, said in a statement on Wednesday.

Two of the seven asylum seekers detained at the Saharonim prison in southern Israel on Tuesday are survivors of torture, the organisations said.

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Meanwhile, hundreds of asylum seekers began a hunger strike late Tuesday in the Holot detention centre to protest the imprisonment of the seven Eritreans, Israeli media reported.

600 deportation notices

There are about 27,000 Eritrean and 7,700 Sudanese asylum seekers in Israel, the UN refugee agency (UNHCR) said.

In November last year, the Israeli government announced plans to deport the remaining asylum seekers, without their consent.

Israel has issued deportation notices to about 600 people to date, the human rights groups said.

The third countries expected to take in the deportees are widely thought to be Rwanda and Uganda, Israeli media have reported, though both African countries have denied making formal deals with the Israeli government.

"It is mind boggling that Uganda and Rwanda agree to take part in this deportation plan and enable Israel to treat African asylum seekers - some of them fleeing from genocide and dictatorship - in this manner," the rights groups said.

In January, the UNHCR urged Israel to reconsider its deportation plan. It said at least 80 asylum seekers deported by Israel between 2015 and 2017 "risked their lives by taking dangerous onward journeys to Europe".

Along the way, "they suffered abuse, torture and extortion before risking their lives once again by crossing the Mediterranean to Italy", said William Spindler, a UNHCR spokesman.