About 1,900 public buses are to be refitted and thousands of trucks will be rerouted around Beijing this summer, in order to keep skies there blue and the air clean, the Ministry of Transport has said.

From next month, all commercial passenger buses entering Beijing will be required to meet national emissions standards, it said.

After a two-month investigation that began in March, the ministry found that 2,610 long-distance buses entering the city failed emissions tests.

"At least 1,890 passengers buses must be refitted to meet the standard, while a further 700 will be replaced," He Jianzhong, ministry spokesman, said.

The government will pay a subsidy of 50,000 yuan for the upgrading of each bus - about half the total cost - and their owners will pay the rest, he said.

Most of the upgrading work has already been completed, he said.

In a further bid to improve air quality during the Olympics, all trucks planning to pass through Beijing at that time will be required to detour via the No 112 state highway, He said.

"The ministry has reinforced bridges and improved the surface of the highway to accommodate the heavier traffic," he said.

According to earlier reports, Beijing officials have promised zero emissions within core Olympic areas. In an effort to achieve that, in March, the government introduced the China IV emission standards, which is equivalent to the Euro IV standard.

Also in March, Wan Gang, minister of science and technology, said more than 500 vehicles used within the core Olympic area - from cars to cranes - will be electric-powered.

Beijing already uses more than 4,000 buses that run on natural gas, which emit just 20 percent of the pollutants spewed out by petrol buses.

Meanwhile, the transport ministry has ordered four ships and a helicopter to patrol the Yellow and Bohai seas as a precautionary measure against possible maritime accidents near Qingdao, He said.

The patrols off the coast of the city - which will host the Olympic sailing events - will continue until after the Paralympics, he said.

Patrols and security drills will also be held every month in Shanghai, Qingdao and Tianjin, he said.