Israel was the "first, fundamental and only suspect" in the suspicious death of Yasser Arafat, a senior Palestinian official said on Friday after receiving reports by Swiss and Russian scientists on samples taken from the exhumed corpse of the late Palestinian leader.

Tawfik Tirawi, who heads a Palestinian committee investigating Arafat's death nine years ago, said he did not die from natural causes, but was evasive when asked repeatedly whether he believed Arafat was poisoned by the radioactive substance polonium-210.

"It is not important that I say here that he was killed by polonium," said Tirawi. "But I say, with all the details available about Yasser Arafat's death, that he was killed, and that Israel killed him."

He later described Israel as the "first, fundamental and only suspect in the assassination of Yasser Arafat".

However, the Russian report said the evidence of polonium-210 was inconclusive. "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 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 the Swiss conclusions, published by al-Jazeera and the Guardian on Wednesday, which said that its tests "moderately support the proposition that the death was the consequence of poisoning with polonium-210".

A third group of scientists, who took samples from the exhumed corpse at the request of French magistrates who are formally investigating Arafat's death in Paris in 2004, has not disclosed its findings.

Israel has vigorously denied any role in Arafat's death, saying it had politically isolated him at the time and had no reason to assassinate him.

"Let me state this as simply as I can: Israel did not kill Arafat," Israeli foreign ministry spokesman Yigal Palmor said on Friday. "The Palestinians should stop this nonsense and stop raising these baseless accusations without any shadow of proof."

It has been suggested that, if Israel was responsible, it must have had assistance from one or more of the 270 Palestinians holed up in the besieged presidential compound in Ramallah where Arafat was effectively imprisoned for two and a half years. Israelis security forces controlled everything that entered the compound, including food and water, but had no control over who consumed which items.

Others have suggested that one or more of Arafat's rivals could have sought his removal.

The Palestinian committee of investigation has made little progress. Arafat's widow, Suha, this week called for a credible and thorough inquiry.

Arafat died on 11 November 2004 at a French military hospital aged 75, a month after falling ill at his West Bank compound. At the time French doctors said he died of a stroke and had a blood-clotting problem, but records were inconclusive about what caused that condition.

Arafat's grave was opened earlier this year, enabling Swiss, Russian and French scientists to take bone and soil samples for separate investigations.

The Swiss team presented its findings on Thursday, saying it found abnormal levels of polonium-210 and lead in Arafat's remains that could not have occurred naturally and that the results "reasonably" support the theory that Arafat was poisoned by polonium. They said the timeframe of Arafat's illness and death was consistent with poisoning from ingesting polonium.

The substance is rare and lethal even in minuscule amounts, and nine years on it would be difficult to track down anyone who might have slipped it into Arafat's food or drink.