Israel may jail illegal immigrants for up to three years under a law put into effect on Sunday, an Interior Ministry official said, a measure aimed at stemming the flow of Africans entering Israel across the porous desert border with Egypt.

"The law takes effect from today," said Interior Ministry spokeswoman Sabine Haddad. The controversial law was passed by parliament in January and denounced at the time by liberal politicians and human rights activists.

Haddad noted that Israeli leaders had said they would try to reduce an influx they view as a threat to Israel's Jewish character, and quoted Interior Minister Eli Yishai as saying last month he wanted to jail or deport illegal immigrants.

Tens of thousands of migrants, mostly from Sudan and Eritrea, have arrived in Israel in the past three years, and the pace has picked up to about 2,000 a month since December, for a total of 60,000 since the influx began.

Many migrants say they are fleeing persecution or war, but few have been granted formal asylum or refugee status, leaving many with temporary visas, subsisting on wages from menial jobs, swelling the ranks of the poor.

"The problem of the infiltrators must be solved and we will solve it," Prime Minister Benjamin Netanyahu said last month, in remarks aimed at calming protests against migrants in Tel Aviv.

Yishai, of the ultra-Orthodox Shas party, told the newspaper Maariv on Friday he saw the African arrivals, many of whom are Muslims or Christians, as a demographic threat.

"The infiltrators along with the Palestinians will quickly bring us to the end of the Zionist dream," Yishai said, adding that Israel had its own health and welfare issues. "We don't need to import more problems from Africa."

"Most of those people arriving here are Muslims who think the country doesn't belong to us, the white man," Yishai said in the interview with Maariv.

Aryeh Eldad, a right-wing lawmaker, urged Israel to order its troops to open fire on any infiltrators who penetrated its borders, rather than only at those suspected of being armed.

Dov Henin, a left-wing legislator, said the measures Israel was taking against the migrants were "immoral," echoing the opinions of Israelis who feel that Jews, often victims of persecution in the past, should show compassion to the migrants.

Some Israelis wondered if jails near the Egyptian border could accommodate all the migrants flowing in.

Israeli media reports said the detention facilities already used to hold thousands of migrants for a number of weeks after crossing the border before they were freed, were not large enough to accommodate more than about 5,000.

Israel has not yet completed a larger facility whose construction was approved late last year, when $167 million was earmarked for it and for a fence being erected along the Egyptian border to block migrants and armed Islamist militants from infiltrating from Egypt's Sinai peninsula.