African migrants in Israel have been told to leave the country voluntarily or be jailed.

The Israeli government has offered migrants up to $4500 to get out of the country within 90 days and have the choice of returning home or another alternative, the BBC reports.

Authorities said the migrants' departures will be handled humanely but they would begin imprisoning from April if they did not choose to leave.

Human trafficking and slave victims, the elderly and children have been excluded from this order.

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The government plans to close the Holot detention centre within three months, according to a statement issued on Monday by Israel’s Population and Immigration Authority.

"[The] authority calls on infiltrators from Sudan and Eritrea to voluntary leave Israel to their country or to a third country," it said.

"Infiltrators who leave Israel by the end of March 2018 will receive a grant of [US]$3,500, payment of airfare and assistance in arranging travel documents."

The authority said the grant's amount would decrease after the deadline passed.

"Enforcement actions will be taken against those infiltrators to which the Population and Immigration Authority allotted a period of time to leave the country, and did not leave," it said.

"Enforcement actions will also be taken against their employers."

The United Nations High Commissioner Refugees said in a statement it was "seriously concerned" about the then-proposal when it was first announced in November by Israel's interior minister Aryeh Deri and public security minister Gilad Erdan.

"As party to the 1951 Refugee Convention, Israel has legal obligations to protect refugees and other persons in need of international protection," UNHCR's Assistant High Commissioner for Protection, Volker Türk said in November.

"UNHCR and the international community have been assisting Israel to meet its international obligations, including by resettling or finding other durable solutions for 2,400 refugees who have departed from Israel in the last couple of years."

According to the UNHCR, there are about 35,000 Eritrean and Sudanese asylum seekers and refugees in Israel.

Since 2009 when Israel took over the process from the UNHCR to determine who is a refugee, only eight Eritreans and two Sudanese have been granted this status.