JERUSALEM — Human Rights Watch on Friday demanded that Hamas end its "cruel and inhuman treatment" of Gilad Shalit as Israel marked the fourth anniversary of the soldier's captivity at the hands of the Islamist movement.

"Hamas authorities are violating the laws of war by refusing to allow Shalit to correspond with his family," the New York-based group HRW said, adding that the young soldier's prolonged incommunicado detention "may amount to torture."

Thousands of yellow balloons were to be released across Israel for the anniversary, a candle-lighting ceremony was planned in Tel Aviv and a major newspaper distributed yellow ribbons to its readers.

Amid growing public backing in Israel for a prisoner exchange deal with the Palestinian Islamist movement Hamas, demonstrations of support for Shalit also have been held in several capitals this week, including Rome and Paris.

On Sunday, the Shalit family accompanied by thousands of supporters will set off on a march from their home in northern Israel to Netanyahu's Jerusalem residence, a distance of about 200 kilometres (120 miles.)

Shalit, then a 19-year-old corporal, was captured by Hamas and other Palestinian armed groups in a deadly cross-border raid from the Gaza Strip on June 25, 2006.

He is believed to be held in Hamas-ruled Gaza, where he has had no contact with his family or the International Committee of the Red Cross.

"Regardless of Hamas?s grievances against Israel, there are no grounds to cut Shalit off completely from his family," HRW said.

"Hamas authorities in Gaza should immediately end the cruel and inhuman treatment of Staff Sgt. Gilad Shalit," it said.

The Islamist movement has said allowing outside access to Shalit could reveal his location to Israel.

HRW pointed out that Israel has prevented detainees from the Gaza Strip from having family visits since Hamas seized power in the Palestinian territory in 2007.

They are however allowed periodic Red Cross visits.

Israel imposed a blockade on the Gaza Strip four years ago a bid to force Hamas to release Shalit, but eased the sanctions this week amid international pressure fuelled by a deadly commando raid on an Gaza-bound activist aid flotilla.

Negotiations for a prisoner swap, mediated by Egypt and Turkey, have hit a brick wall.

Hamas wants Israel to release hundreds of prisoners, including several top militants responsible for killing scores of Israelis, in exchange for Shalit -- a price the Jewish state has been reluctant to pay.

A majority of Israelis are in favour of such a swap, according to a poll published by the Yediot Aharonot daily on Friday.

Seventy-two percent said yes when asked if they would support "a prisoner exchange deal in which hundreds of terrorists, including murderers, are released in exchange for Gilad Shalit."

The largest Israeli newspapers devoted front pages and several inside pages to the anniversary, with Maariv featuring an emotional appeal from Shalit's mother and Yediot Aharonot splashing the headline: "Gilad we love you."

Public radio devoted most of its early morning news magazines to the commemoration of Shalit's capture.

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