JERUSALEM — Israel’s parliament on Tuesday approved harsh new penalties on illegal migrants and Israelis who help them, building on other contentious measures designed to staunch the flood of Africans seeking sanctuary here.

The bill allows imprisonment of illegal migrants for an unlimited time without trial. People caught helping them could face prison terms of up to 15 years.

Critics deplore the new law as an unconstitutional trampling of human rights. They accuse the government of failing to formulate a coherent, humane policy on illegal migration that would address an issue that has become increasingly urgent over the years.

“I am not familiar with a Western country that allows such a law,” said Asaf Weipzen, a lawyer for the Hotline for Migrant Workers, an advocacy group. Detentions in other countries are finite, and the European Union, for example, has an 18-month limit on incarceration, Weipzen said.

Africans began trickling into Israel through its porous southern border with Egypt’s Sinai desert after Egyptian security forces violently quashed a demonstration by a group of Sudanese refugees in 2005.

The number of migrants surged as word spread of safety and job opportunities in the relatively prosperous Jewish state. The government estimates 50,000 Africans — the overwhelming majority from Sudan and Eritrea — have illegally entered Israel since.

Prime Minister Benjamin Netanyahu, who has called the swelling number of illegal migrants a “national scourge,” voted for the bill.

His spokesman, Mark Regev, called the legislation part of a “multi-tiered strategy to deal with the challenge of illegal immigration to Israel.” He would not comment on critics’ concerns about the new law.

Israel already has repatriated hundreds of Africans and Netanyahu has said he would explore the possibility of repatriating others when he visits Africa this year.

Last month, the Cabinet voted to finance a $160 million program to finish building a 150-mile (250-kilometer) border fence along the Egyptian border and expand detention facilities to hold thousands of new arrivals. Employers who hire illegal migrants now face stiffened fines of up to $18,000.

Lawmaker Amnon Cohen, of the ultra-Orthodox Shas party, said he sponsored the latest bill “to defend Israeli society.”

“Go to south Tel Aviv and you’ll see people there living in fear,” Cohen said. “Anyone who wants to steal a wallet from a person, or a box of goods from a store or a bike from a private garden just does whatever he wants. If we don’t put an end to this, the issue will not stop.”

Migrant advocates contend the Africans are bona fide refugees and should be granted asylum. They accuse the government of failing to process asylum applications and ignoring the retribution most migrants would face should they return home.

William Tall, the U.N. refugee agency’s envoy in Israel, said his agency “continues to have serious concerns that the law does not sufficiently state that asylum seekers and refugees are exempt.”

Some believe the migrants are an economic and social burden and fear their mounting numbers will dilute Israel’s Jewish character.

“If we don’t put an end to the phenomenon of infiltrators, all of Israel’s social systems will collapse,” Netanyahu’s Cabinet secretary, Tzvi Hauser, told Israel Radio.

Others say the Jewish people, because of their history of persecution, must be especially accommodating of others escaping persecution or conflict.

“This law contradicts Israel’s obligations according to international law, it goes against the human norm of helping refugees running for their lives, and it flies in the face of the Jewish principles of honoring the stranger among us,” said lawmaker Dov Khenin of Israel’s leftist Hadash Party.

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