As President Bush prepares to take his case against Iraq to the United Nations this week, chief U.N. arms inspector Hans Blix says he has no evidence that Saddam Hussein is getting ready to produce nuclear bombs. But Mr. Blix, who briefed the Security Council in New York Tuesday, says only on-site inspections can rule it out categorically.

The chief U.N. arms inspector says he does not have what would constitute proof of an Iraqi arms build-up. But neither does he rule out the possibility. He says Iraq needs to provide more information to weapons experts, which then would have to be analyzed before anyone could be quite sure of what exists in the country:

"If I had solid evidence that Iraq retained weapons of mass destruction or were constructing such weapons, I would take it to the Security Council, report to them," he said. "However, there are many open questions. These questions need to be answered. Many of them have not been so, and it will be one of our tasks in getting back to Iraq to discuss these issues with them."

As for satellite pictures the Bush administration says point to new and suspicious construction in Iraq, Mr. Blix says the photographs can be helpful but not conclusive.

"One can see whether there have been extensions, whether they are re-built, etc. But this is not the same thing as saying that there are weapons of mass destruction," he said. "It is precisely for these reasons that you would like to have inspectors on the ground, so you can view horizontally, not just vertically. The satellites do not see through the roofs."

It is not clear what the Security Council will do to toughen its approach to Iraq. One possible outcome is setting a deadline for letting the inspectors back in, with a threat of punitive action if Iraq fails to comply. Iraq banned inspection teams nearly four years ago.

Iraq continues to maintain that it is not producing weapons of mass destruction. But with the difficult history between Iraq and the United Nations, Council diplomats do not take Iraq's word as a guarantee.

At the same time, U.N. diplomats are trying to dissuade the United States from taking unilateral action against Iraq. The Bush administration has been debating a military strike to remove Saddam Hussein from power.