Failure to do so could see Iran's case taken before the United Nations Security Council where the Islamic Republic would face possible economic and diplomatic sanctions.

"At the moment it looks like they're on a collision course with the Security Council," said one European diplomat in Tehran. "You can't rule it out, but I can't see them pulling a surprise and meeting our demands before the deadline."

International Atomic Energy Agency (IAEA) inspectors are due in Tehran Thursday to begin a crucial round of visits and talks to verify Iran's statements that its sophisticated network of nuclear plants is geared solely to electricity production.

IAEA chief Mohamed ElBaradei said Tuesday Iran must give his team "full cooperation" soon or face accusations it is secretly building a nuclear weapons capability.

But as the clock ticks Tehran shows little sign of softening its stance.

Angered by the IAEA call for it to clear up remaining questions and halt uranium enrichment, Iran says it will only give inspectors limited access to nuclear sites.

Enriched uranium can be used as fuel for power plants, or as bomb material if highly enriched.

Iran says arms-grade enriched uranium traces found at two Iranian plants by IAEA inspectors this year were caused by contamination from imported centrifuge enrichment parts.

But to the dismay of the IAEA, diplomats say, Iran still refuses to say where the imported parts came from.

"They just don't show any sign of changing their pattern of very grudging and limited cooperation," a diplomat said.

Hard-liners, who control the main levers of power in Iran and tend to have the final say on security matters, argue Tehran must not bow to international pressure.

"Pulling out of the NPT (nuclear Non-Proliferation Treaty) is the most logical path to adopt," Hossein Shariatmadari, influential editor of the hardline Kayhan newspaper, wrote in an editorial Tuesday.