By Noah Browning

RAMALLAH, West Bank (Reuters) - A Russian report quoted by Palestinian investigators on Friday said there was insufficient evidence to support the theory that Yasser Arafat died in 2004 by polonium poisoning.

The findings were far weaker than those of a Swiss laboratory announced with fanfare on Wednesday by Arafat's widow and Qatar-based al-Jazeera television.

Palestinians remained unfazed by the results - which dampen but don't definitely disprove the possibility of death by poisoning - and officials continue to blame arch foe Israel.

Samples were extracted from Arafat's corpse last November by Swiss, French and Russian experts after an al-Jazeera documentary revealed unusually high amounts of the deadly Polonium isotope on his clothes.

"The outcome of the comprehensive report on the levels of Polonium-210 and the development of his illness does not give sufficient evidence to support the decision that Polonium-210 caused acute radiation syndrome leading to death," said Dr. Abdullah Bashir, quoting the conclusions of the Russian report.

But Dr. Bashir said that both the Swiss and Russian reports found "large amounts" of the radioactive isotope in his remains.

The Russian findings were significantly more cautious than Swiss conclusions which said that its tests "moderately support the proposition that the death was the consequence of poisoning with Polonium-210", though the evidence was not conclusive.

Arafat, a guerrilla leader who became the Palestinians' first president affectionately known by the nom de guerre Abu Ammar, contracted a sudden and mysterious illness while surrounded by Israeli tanks in his compound in Ramallah.

Palestinians have long blamed Israel for his death - a charge it denies - but a local investigation committee has made little tangible progress in explaining the case.

"Palestinians should stop leveling all these groundless accusations without the slightest proof because enough is enough. We have strictly nothing to do with this and that is all there is to it," Israeli foreign ministry spokesman Yigal Palmor told Reuters.

"MARTYR'S SOUL"

Tawfiq Tirawi, intelligence chief at the time of Arafat's death and now head of the Palestinian committee, began Friday's press conference at the compound where Arafat first fell ill with a prayer for what he called "the martyr's soul".

"Abu Ammar did not die of age, or sickness or from a natural death," Tirawi said.

"Our efforts are ongoing ... to find out who stands behind the death of Yasser Arafat and who has the technical and scientific means for this. We consider Israel the first, fundamental and only suspect in Yasser Arafat's assassination."

He brushed off as "rumors" speculation by some Palestinians that members of Arafat's entourage killed him, saying his committee deals only in facts and evidence.

Arafat's widow, not mentioning Israel at all, explicitly accused members of his "close circle" in a Reuters interview.

Israel and the Palestinians resumed peace talks in July to try to resolve their long conflict and live side-by-side in two sovereign states.

(Editing by Ralph Boulton)