The U.N. nuclear watchdog is locked in a ``stalemate'' with Iran over the country's suspect nuclear programme, the agency's chief said on Monday, pressing Tehran to answer lingering questions about its atomic ambitions.

In a statement to the 35-nation board of the International Atomic Energy Agency (IAEA) — which is taking a hard new look at Iran and Syria this week — Mohamed ElBaradei urged the Islamic Republic to “substantively re-engage'' with the international community.

Iran insists its nuclear programme is peaceful. The United States and key allies contend it is covertly trying to build a bomb.

This week's meeting in Vienna, and the upcoming U.N. General Assembly, could set the stage for a toughening of sanctions against Iran for its continued defiance of Western demands that it suspend uranium enrichment. Tehran already has defied three sets of U.N. Security Council sanctions.

Mr. ElBaradei acknowledged that Iran has provided IAEA inspectors access to a research reactor at Arak and has tightened security at its main nuclear facility in the southern city of Natanz. But he said Iran is still enriching uranium, which can be used for nuclear fuel or — if enriched to a high enough level — can produce fissile material for a warhead.

``On all other issues relevant to Iran's nuclear programme ... there is (a) stalemate,'' Mr. ElBaradei told the IAEA board.

``Iran has not suspended its enrichment-related activities or its work on heavy water-related projects as required by the Security Council, nor has Iran implemented the Additional Protocol,'' which would open its nuclear facilities to unannounced and more intrusive inspections.

``It is essential that Iran substantively re-engage with the agency to clarify and bring to closure all outstanding issues,'' he said.

``Iran needs to respond fully to all the questions raised by the agency in order to exclude the possibility of there being military dimensions to its nuclear program,'' he added.

Mohammad Saeedi, deputy head of the Atomic Energy Organization of Iran, said his country has provided the IAEA with all the information it needs and that it was now up to the agency to act.

``In our view, we believe that the agency should finalize the outstanding questions,'' Mr. Saeedi said.

Mr. Saeedi said Iran has also ``cooperated widely'' with the IAEA to improve safeguard measurements at Natanz and ``voluntarily'' provided U.N. nuclear inspectors access to its Arak site.

Mr. ElBaradei also criticized Syria for failing to disclose details about a desert site bombed by Israel in 2007.

``Syria has not cooperated with the agency to confirm Syria's statements regarding the non-nuclear nature of the destroyed building on the Dair Alzour site, nor has it provided the required access to information, locations, equipment or materials,'' he said.

Mr. ElBaradei had a terse assessment of North Korea, which conducted its second nuclear test explosion in May. The IAEA pulled out its inspectors after North Korea suspended all cooperation with the IAEA in April, and since has been unable to monitor or verify Pyongyang's nuclear activities.

``I therefore have nothing to report to the board,'' he said.

Before the meeting, Iranian President Mahmoud Ahmadinejad said his country will neither halt uranium enrichment nor negotiate over its nuclear rights, but is ready to sit and talk with world powers over ``global challenges.''

``From our point of view, Iran's nuclear issue is over. We continue our work within the framework of global regulations and in close interaction with the International Atomic Energy Agency,'' he said. ``We will never negotiate over obvious rights of the Iranian nation.''

``We cooperate with IAEA within the framework of regulations but we will resist if the agency is influenced by political pressures,'' he added.

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