The head of the United Nations nuclear watchdog has reacted with fury at the United States' delay in passing on intelligence, after Washington accused North Korea of helping Syria to build secretly a nuclear reactor.

According to the Central Intelligence Agency the reactor, which may have been part of a nuclear weapons programme, was destroyed by an Israeli air raid months before its completion.

Mohamed ElBaradei, the International Atomic Energy Agency chief, issued a stiffly worded statement in which he criticised the US and Israel, as well as Syria, which should have informed the IAEA if it was building a nuclear reactor.

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Noting that the IAEA was only informed by the Bush administration about the nuclear reactor claim on Thursday, seven months after the Israeli raid, he deplored the fact that the information had not been provided to the agency in a "timely" manner.

Mr ElBaradei also pointed out that, according to the information provided by the US, "the reactor was not yet operational and no nuclear material had been introduced into it". According to a Western diplomat in Vienna, that meant that if the UN inspectors had been alerted earlier, they would have been able to verify the facts on the ground. "Now nobody is ever going to know for sure," the diplomat said. In his statement, Mr ElBaradei criticised Israel for its "unilateral use of force" which undermined the "due process of verification".

Mr ElBaradei was said to be incensed by the American failure to inform the UN watchdog about the possible risk of nuclear proliferation when the Americans became aware of the nature of the site in 2006. "What kind of non-proliferation regime is this, when they come to the IAEA months after it's been bombed?" the diplomat said. The IAEA chief pledged to investigate the US information "with the seriousness it deserves".

The images that were released to the press after a closed-door briefing to US Congressional panels raised new questions as the date of the alleged reactor pictures was not clear.

The head of the Syrian nuclear commission was shown standing next to a car with a Syrian number plate with the head of North Korea's Yongbyon reactor, which has made weapons grade plutonium. But there was no indication of the date. American officials told reporters that their suspicions were raised because of the secrecy surrounding the project, in comments that had echoes of the Iraq weapons of mass destruction fiasco.

Israel, the only nuclear power in the Middle East, refused to comment on the matter. Syria continued to deny the charges as "ridiculous", although the reactor images were described as "compelling" by the American nuclear expert David Albright. But he added that the lack of other facilities "has to give pause before accusing Syria of having an active weapons programme".

Syria accused the Bush administration of having been "apparently party to the execution" of the air raid. An American official said Washington did not give Israel any "green light" to strike the area, although The New York Times reported that there had been extensive discussions between the US and Israel before the raid on 6 September last year.

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"It's sheer fabrication," said the spokesman for the Syrian embassy in London, Jihad Makdissi, adding that the reactor images showed a deserted military building. "The CIA giving testimony is the same CIA that briefed Colin Powell who spoke at the UN Security Council about Iraqi weapons of mass destruction."

Asked about the photograph of the scientists, Mr Makdissi pointed out that "Syria and North Korea have economic relations. This person was not smuggled into Syria – did you see a nuclear rocket in his hands?"

The impact on North Korea's nuclear disarmament commitments at six-party talks remained unclear last night. The chief US negotiator, Christopher Hill, said that the co-operation between Syria and North Korea was no longer active, suggesting that the Bush administration had already turned the page.

The need for more answers

*Questions for Syria and N Korea

Why does Syria continue to deny that a reactor was being built despite the images presented to Congress?

Why does North Korea deny the accusations?

Where and why did the head of the Syrian atomic energy commission meet the North Korean nuclear scientist?

Why did Syria not keep the International Atomic Energy Agency informed of the reactor progress in line with IAEA regulations?

Why did Syria not respond after the Israeli strike?

Why did Syria continue its back-channel talks with Israel after the raid?

*Questions for the US and Israel

Why did the US wait until last Thursday to inform the IAEA about the reactor when it had been aware of the site possibly as early as 2006?

Why did it take the CIA seven months to reveal this information to Congress?

Does the intelligence show any intention to build a weapon?

Is there any directevidence of North Koreans actually at the site? The photograph of the Syrian and North Korean nuclear scientists was taken at another location

Why did Israel act unilaterally to destroy the site before IAEA inspectors had a chance to visit?

To what extent was the US involved in the Israeli air strike?

When did Syria and North Korea stop their nuclear co-operation, which the US says has been halted?