BEIJING — On a day when an International Olympic Committee medical authority reconfirmed his belief air quality will not pose a major problem for athletes at the Beijing Olympics, a group of U.S. cyclists was taking precautions and a Portuguese cyclist withdrew.

Mike Friedman and a small group of American Olympians wore face masks in the baggage claim area upon arriving yesterday afternoon at the airport.

"It's really just taking every precaution necessary," Friedman told the Associated Press.

Portugal's Sergio Paulinho dropped out. His coach told Reuters the cyclist has a respiratory problem that can get worse in Beijing because of the pollution. Arne Ljungqvist, the chief medical commissioner for the International Olympic Committee, said yesterday morning, "The misty air is not a factor of pollution primarily but a feature of evaporation and humidity."

The polluted air has been worrisome for the IOC. Local organizers promised they would clean up the air when they were awarded the Games seven years ago. Despite efforts to reduce automobile traffic and close factories, the past three mornings have been veiled in white. Ljungqvist said he did not think it was necessary to wear masks. "We cannot see health risks for athletes."

The IOC monitors the government's hourly analysis of air conditions. Plans exist to postpone or cancel events.

Friedman's teammates were not identified. "You got to take every chance you have just to protect the airways," Friedman, 25, said.

In the Kashgar region yesterday, China offered apologies yesterday for the beatings that police gave two Japanese journalists who were covering a deadly assault by Muslim separatists.

Paramilitary police kicked and beat the journalists, throwing one to the ground, putting boots to his head and body, and damaging his photo gear.

In a separate incident, police entered the hotel room of an Agence France Presse photographer and forced him to delete photos of the attack scene, the French agency said.

The treatment drew sharp protests from the government of Japan, and Japan's Embassy in Beijing said in a statement that the Foreign Ministry had written a letter expressing regret for the incident.

A top Communist Party leader in this gas-rich region, meanwhile, said assailants had left literature at the scene of an attack a day earlier pledging "holy war" in China. In the attack, two minority Uighurs are suspected of commandeering a truck and plowing it into a platoon of jogging policemen, killing 16 and injuring another 16, four of them seriously, authorities said.

Shi Dagang, the party secretary for Kashgar prefecture, said the attack was "a well-thought-out and long planned" assault by Muslim separatists who want to sever the Xinjiang autonomous region from China and create an independent East Turkistan.

"Their aim is to use simple means to attack the Chinese government and turn the year 2008 into a year of mourning," Shi said.

He identified the two arrested Muslims as a 28-year-old vegetable peddler and a 33-year-old taxi driver. He said nine homemade bombs and a homemade gun were found at the scene of the attack and that the assailants were part of a "terror group" with tentacles abroad.

USA Today and McClatchy-Tribune News Service contributed to this report.