The uncle of a missing Israeli teenager has made a heartfelt plea for "mercy" to his suspected kidnappers, after the family spent a fourth sleepless night worrying for his safety.

Naftali Frenkel, 16, who holds joint Israeli and US citizenship, is one of three teenagers Israel says were kidnapped last week from the southern West Bank by militants from the Islamist Hamas movement.

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Netanyahu pins kidnap on Hamas
• Video: Missing Israeli boy's family pleads for safe return

On a leafy residential street in Nof Ayalon, a small town some 20 kilometres northwest of Jerusalem, police and television cameras vie for space outside the Frenkel family home.


"I'd just like to send a message to anyone who listens... about human mercy, about a belief I think all of us share in God above... (and) mercy to young children," Yishai Frenkel told AFP.

"The abduction of 16-year-olds is a terrible crime," he said.

"There are lines that should not be crossed."

Frenkel said the family first realised Naftali had gone missing early on Friday and has slept very little since then.

Israeli Prime Minister Benjamin Netanyahu has said he holds Palestinian president Mahmud Abbas, who recently signed a unity deal with Hamas, responsible for the safety of Frenkel and the two other missing youths: 15-year-old Gilad Shaer and 19-year-old Eyal Ifrach.

The three teenagers were first reported missing late on Thursday but so far there has been no formal claim of responsibility.

Israel has arrested more than 150 Palestinians in one of the biggest sweeps in recent years, with the searches focusing on the southern West Bank city of Hebron and the surrounding area.

The teens' disappearance has captured the hearts of the nation, with some 20,000 Israelis pouring into the Western Wall plaza in the Old City of Jerusalem - the holiest site at which Jews can pray - in a mass prayer late on Sunday for their safe return.