(Updates with Israeli army statement, detail)

By Alastair Macdonald

GAZA, April 20 (Reuters) - The Israeli army said on Sunday it was investigating the death of a Reuters cameraman in the Gaza Strip last week and declined to say why one of its tanks opened fire, killing the journalist and five other Palestinians.

Four days after 24-year-old Fadel Shana was hit by a spray of metal darts from a controversial type of missile, the international news agency welcomed the response to its demands for an inquiry and urged the army to present its findings speedily.

Two teenagers died on Sunday of wounds sustained in the incident on Wednesday, raising the death toll to six.

The Israel Defence Forces said: "The IDF is conducting a field investigation to look into the claims regarding the circumstances of the death of a Reuters cameraman."

It added that it investigated all instances of civilian deaths and that all such field investigations were passed to the armed forces' senior law officer, the Military Advocate General.

Reuters Managing Editor for the Middle East, Mark Thompson, said: "We welcome this investigation and urge the IDF to share its findings with us as soon as possible."

Dozens of civilians have been killed in the Gaza Strip this year as Israeli forces have confronted Islamist militants from the enclave's ruling Hamas movement and other factions who have fired rockets into Israel and mounted other attacks on it.

Nineteen Palestinians were killed on Wednesday, 13 of them described by medics as civilians, after militants killed three Israeli soldiers who were on an raid into the territory.

It was the army's costliest day in Gaza in the past year.

"KILL TERRORISTS"

Israeli television screened one commander telling his men as they prepared to mount a new raid afterwards: "Four wounded and three dead in an encounter with three terrorists -- so we have been given a mission to go in ... Of course, we will try to locate terrorists and to kill as many terrorists as possible."

Data was not immediately available on how many field investigations the army has conducted into such deaths or whether any had led to disciplinary action.

The army statement rejected what it described as claims that it deliberately targets civilians. New York-based Human Rights Watch has said it has evidence suggesting that the tank fired "recklessly or deliberately" at the Reuters news crew.

Hospital officials named the two youths who died of their wounds on Sunday as Ahmed al-Najar, 17, and Bilal al-Dhaimi, 16.

Human rights activists and local residents have given the ages of three men and youths killed instantly alongside Shana as between 13 and 22.

Shana's body armour, which bore a blue-on-white "PRESS" marking, was ripped off by the attack, which medical examination showed had thrust several 1.5-inch (38-mm) metal darts through his neck, shredding his flesh and severing his spine.

He was filming a panoramic view of an area where an Israeli air strike had earlier killed several people at a spot about 1.5 km (a mile) from where he was standing. Next to him was his car, a sport utility vehicle bearing "TV" and "Press" markings.

A tank was also visible in his film, about 1.5 km away. It fired a shell seen exploding in mid-air, consistent with rounds designed to spray people with darts, known to the military as flechettes. Moments after that explosion, the camera stopped.

The Israeli army has defended its use of flechettes, noting that the Israeli Supreme Court turned down a petition to ban their use as a danger to civilian bystanders.

Shana's soundman, Wafa Abu Mizyed, was hit in the wrist. (Additional reporting by Nidal al-Mughrabi, editing by Ralph Boulton)

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