IOC president Jacques Rogge marked Monday's milestone , 500 days before the Beijing Olympics,by calling for a strong focus on this summer's 26 test events.

Though countdown clocks across Beijing will show the milestone, events in China's capital to mark the moment were set to be low-key. The big day will be August 8,the one-year mark,which will also signal the start of several months of test events.

"I have learnt through my Olympic experience that test events are the key to the success of the Games," said Rogge, president of the International Olympic Committee.

"Great progress in venue construction should not mean that the organizers lose focus on other areas of Games preparations. Because it is these moments when you lose your concentration that mistakes can creep in."

To mark 500 days, Rogge gave written replies to a series of questions ranging from venue construction, to test events and air quality.

Construction on roads, stadiums and other venues is far ahead of schedule, and Rogge said the building of 31 Beijing-based venues "was progressing at a tremendous pace."

Rogge will be in Beijing in late April for IOC executive board meetings, following a visit last October.

"When walking through the streets of Beijing, the pride and enthusiasm of its people is palpable," Rogge said.

Dirty air and chaotic traffic have been a major concern of Games organizers. Rogge said both issues would top the agenda when the IOC's coordination commission meets with local organizers in Beijing in mid-April.

Beijing organizers have mostly shunned foreign expertise. However, with just over 16 months to go this may be changing. Rogge said the test events would show the level of preparation.

"Should issues arise, it is important to have the systems in place to identify and assess those issues," Rogge said.

"It is important, however, that these (test) events are not treated on their own but as part of an integrated preparation for the Games," he added. "Because it is through learning the lessons from these events that BOCOG (Beijing Organizing Committee for the Olympic Games) as an organization will achieve their objective of hosting successful Olympic Games in 2008."

Rogge lauded Beijing officials for moving some heavy industry _ steel and chemical plants _ out of Beijing, and for planting 533 hectares (1,320 acres) of tree and shrubs around the airport and Olympic venues. He also said 90 percent of Beijing's sewage was now being treated at new plants.

"I think it is also important to remember that environmental challenges are not new to the Olympic Games," Rogge said.

"In Athens, for example, there were issues of heat, and the Los Angeles and Seoul Olympic Games had challenges with air quality."

Beijing has 2.9 million registered vehicles, and the number is expected to reach 3.3 million by the Olympics, a 13 percent increase. This is a side effect of China's 9 percent growth rate over 25 years, and has also produced US$40 billion (euro30 billion) building project to modernize China's capital.

The Olympics venues are the catalyst for some new construction, but they make up a minuscule portion of the construction around the capital, where a new skyscraper is topped out each week.