The U.S. officer overseeing NATO's preparations to help guard the Olympic Games said he has seen no specific intelligence information signaling a possible terrorist attack in Greece during the games this month.

"There's sort of a baseline hum you get in certain areas of terrorist activity," said Adm. Gregory G. Johnson in his first U.S. interview about NATO's role at the Olympics. In the case of Greece, "the noise level, shall we say, is pretty benign right now, and I just don't see any spikes."

Nonetheless, Johnson said the absence of such threat indicators is no cause for complacency, especially for an event as significant -- and as inviting a terrorist target -- as the world's largest gathering of athletes.

"We don't want to be too comfortable," he said. "There could be something deeply buried that we just haven't seen or anticipated."

Indeed, there were signs here last week at the headquarters of NATO's southern Joint Force Command that alliance authorities were continuing to plan for the worst.

Inside a bunker known as Tunnel 4, built into a hillside behind a complex of NATO office buildings, military specialists from a handful of alliance countries spent much of last week practicing responses to a series of massively damaging potential attacks. Seated at a long table filled with computer screens, the officers examined the likely effects of the release of chemical or biological agents or explosions of radiological devices at assorted locations in Greece, then assessed how fast NATO forces could get to the scene and from where.

But for all such doomsday rehearsing -- and headlines in recent weeks about NATO bolstering security at the Olympic Games -- the alliance's role appears destined to be limited.

NATO's involvement remains a sensitive issue for the Greek government, which has made clear it believes security is primarily its responsibility. Only after the bombing of trains in Madrid that killed nearly 200 people in March did Greece approach NATO for help, and that request was confined to air and maritime patrols and a 200-member unit that specializes in coping with the aftermath of a chemical, biological or radiological attack.

At the urging of the Bush administration, which wanted to add U.S. military commandos to the mix, Greece last month went a step further and asked NATO also to provide several hundred Special Operations Forces for rapid-reaction, counterterrorism missions. That request was controversial not just in Greece but also within the alliance, where some members -- most notably Germany -- treat counterterrorism activity as a civilian law enforcement responsibility, not a military one.

"Normally, counterterrorism missions have always been done on a bilateral basis, and this is the first time that NATO has taken on the mission," said U.S. Marine Gen. James L. Jones, NATO's top military officer.

Although alliance ambassadors approved the move last week, NATO officials said details on where to position the forces are still being negotiated with Greece. The forces will come from U.S. ranks and report to the U.S. officer heading all NATO operations related to the Olympic Games, Army Maj. Gen. Rick Lynch.

"We will have various basing modes, I'm sure, to include sea-basing and to include locations outside of Greece," Jones said in a telephone interview.

Other elements of NATO's involvement in the Olympics are being portrayed by alliance officers here as routine for high-profile international events, or a slight variation of alliance operations.

The AWACS surveillance aircraft that will be in the skies over Greece, for instance, also flew over recent summits of NATO and Group of Eight leaders. The fleet of eight ships that will patrol international waters outside the Aegean Sea, looking for suspicious craft that might be headed toward Greece, is the same NATO naval operation set up in October 2001 after the Bush administration's declared war on terrorism to monitor ship traffic in the Mediterranean.

In a concession to Greece, the commander of this naval force will be Greek. Under the alliance's usual rotation schedule, the officer had not been due to replace the current German commander until September. But the change of command was accelerated to Aug. 5 with Germany's agreement, in time for the Games, which begin Aug. 13.

"To have a commander of the naval force who knows Greece well is advantageous, although NATO has been benignly neutral in this case," said Vice Adm. Ferdinando Sanfelice di Monteforte, who heads NATO maritime operations in southern Europe. "The matter was dealt with by Germany and Greece."

With the chances of an air or sea attack considered even slimmer than something occurring on land, NATO appears positioned at the Olympics less to help stop potential trouble than to help deal with its aftermath.

"There's not a lot NATO can do against what is essentially a high-grade, law enforcement problem," said E. Wayne Merry, a former diplomat and specialist in counterterrorism at the American Foreign Policy Council. "NATO's primary contribution would be in providing relief if there's a major attack."

Mindful of Greek sensitivities, Johnson emphasized that NATO would be playing a supporting role to the Athens government, which is deploying more than 70,000 security personnel and an array of devices, including hundreds of street surveillance cameras, several Patriot antimissile batteries and two mobile X-ray scanners for checking trucks and cars.

"If the question is, do I feel constrained or concerned about the constraints, the answer is no," Johnson said. "Greece has been more than generous and fully cooperative. We're very happy to be able to help in a very, very small way."

In preparing for security at Athens this month, an anti-terrorist unit of Greek police performs an underwater drill.For the Olympic Games, Hungary contributed to NATO security forces this mobile biological laboratory designed to detect terrorist attacks.