With little fanfare, the United Nations has dramatically increased its effort in Iraq to detect stockpiles and production sites of weapons of mass destruction still hidden by Saddam Hussein's regime.

Late last week, the U.N. began unannounced helicopter flights to suspected sites for the first time in the effort to decapitate the mass-weapons complex. The intensified inspections were initiated because inspection teams are running out of intelligence leads and time and still have not found all Saddam's nuclear, chemical, biological and missile sites.

The inspectors' success has always depended on intelligence, such as a defector's tip or a satellite photo that triggered a site visit. But as Iraqi concealment has intensified, such leads have dried up. "I'd send in a team [every] week if I could," a U.N. official told me recently, "but I don't know where to tell them to go."

And as the visits yield ever more meager results, pressure is building within the U.N. to stop looking for new sites and simply monitor what's already found.

The International Atomic Energy Agency (IAEA), which visits the sites the U.N. designates, is part of this pressure. "Practically the largest part of Iraq's nuclear program has now been identified," said Maurizio Zifferero, who leads the IAEA's Iraq visits. "Probably what is missing is just details."

But Rolf Ekeus, who heads the U.N. inspection effort, disagrees. He has repeatedly said that vital parts of Saddam's nuclear program are still to be found.

U.N. inspectors think Iraq has built an undetected experimental array of centrifuges called a cascade to purify uranium to weapons grade level. The inspectors are also looking for missiles. They know that of the 819 Scud missiles Iraq bought from the Soviet Union, 487 were fired in battle, used in tests, or otherwise destroyed, including 93 fired in the Persian Gulf War. But Iraq refuses to reveal launch records, so the overall number of expended rockets cannot be verified. The CIA is known to believe 200 or more Iraqi Scuds are still hidden.

Part of the cause for concern is the recent belief in Washington that despite Pentagon claims of having destroyed numerous mobile Scud launchers and support vehicles, it is now thought that U.S. Air Force fighter-bombers failed to bomb any Scud missiles during Desert Storm's air campaign. Neither U.N. inspectors nor Pentagon spokesmen can now confirm that a single operational Iraqi Scud was it by an American bomber. U.S. pilots did destroy some fixed Scud launchers in the desert, but no missiles were near them.

The U.N. inspectors are also looking for a second Iraqi missile, the Badr-2000, better known as the Condor II. It can fly 600 miles, three times farther than the Scud, and is big enough to carry nuclear warheads. U.N. inspectors report finding the factory where the first stage was built but have no information about second stage or guidance system production.

Egypt and Argentina, which joined with Iraq to develop the missile in the 1980s, could shed light on the program, but inspectors say they have received no help.

Chemical weapons are also a worry, but the inspectors found that Iraqi nerve gas was only 2 to 7 percent pure (by comparison, U.S. gas is over 90 percent pure) and degrades rapidly. But U.S. intelligence sources say manufacture of biological weapons, which unlike lethal chemical weapons can be produced in small spaces without elaborate apparatus, may already have resumed at sites the U.N. has not visited since last summer. There are also thousands of buildings and bunkers in Iraq that have never been inspected. The invasive new tactics attack this problem.

The plan's advocates envision inspectors living in Baghdad semi-permanently instead of the customary periodic arrivals on two days' notice. From there they can make daily helicopter flights to suspected weapons sites, making concealment and manufacture more difficult.

Only two or three inspectors have moved to Baghdad, with more likely to follow. U.N. officials worry about questions of control and logistic support. The U.N. now has secure telephones, so New York managers can talk to inspectors in the field. But the inspectors have complained that Iraq's penchant for electronic bugging makes secure communications impossible, either among themselves in their hotels or with New York.

The Bush administration wants the U.N. to set up a secure office complex near an Iraqi airport, with support personnel for a score or more inspectors and rooms shielded from bugging. The U.N. has already moved 35 German airmen and their helicopters to Baghdad, enough to keep several inspectors flying every day. "The Iraqis," said one inspector, "really hate the helicopters."

U.S. officials say that a score of inspectors in Baghdad could saturate the few areas where there is enough industrial infrastructure for Iraq to mount a major missile or nuclear production. The theory is that continuous inspection would force Iraq to move equipment -- and be detected.

Thus, Saddam could be convinced that continued resistance to the inspections will only prolong the embargo for nothing.

Iraq still rejects the U.N. Security Council resolutions requiring full disclosure and long-term monitoring of its mass-weapons programs. Baghdad recently filed what it called "full, final and complete" disclosure, but the inspectors found little new in it. They are certain there is a lot more to be found.