BEIJING — China's efforts to stop the spread of its deserts are reducing the severity of sandstorms like the one that dumped yellow grit as far away as Japan this week, but the problem cannot be entirely controlled, officials said Thursday.

"The situation is still rather grave," Liu Tuo, deputy director of the Chinese State Forestry Administration's sandstorm-control agency, said at a news conference.

China has planted thousands of acres of vegetation to stop the spread of deserts in its north and west, and Liu said it has succeeded in reversing the process in some areas.

But it will be decades before the effort is complete, said Liu and Yang Weixi, the agency's chief engineer. They also said the worst storms are beyond China's control because they start in Russia or Mongolia.

The storm that hit Beijing this week, dumping 300,000 tons of dust and sand on the Chinese capital, reportedly was the most severe in at least five years. Dust was blown as far away as South Korea and Tokyo.

Spring wind storms blanket Beijing in choking yellow dust every year.

This spring's storms were unusually bad due to very dry conditions after rainfall over the past year was up to 80 percent below normal levels, said Liu. He and Yang said the Chinese capital has been hit by 10 dust storms since February.

China's dust storms were at their worst in the 1950s and '60s after campaigns to raise farm and factory output following the 1949 communist revolution stripped the soil of vegetation in many areas.

Conditions have improved markedly since then, despite the severity of recent storms, Yang said.

Beijing has approved programs to reclaim land by planting hardy grasses and shrubs on 30 percent of China's 700,000 square miles of desert, Liu said.

But he said that wasn't due to be completed until 2050 and given the millions of square miles of desert in China, "they will continue to be a source of sandstorms."

The government also has planted "green belts" of trees to shield Beijing and the neighboring port city Tianjin from dust and sand sweeping down from the northwest.

Liu expressed confidence that sandstorms would not disrupt the 2008 Beijing Olympics -- a key prestige project for communist leaders. They have promised an environmentally sound "green Olympics."

Liu noted that Beijing's sand and dust storms usually are confined to March, April and May, while the Olympics is planned for August.

"Even if there are sandstorms in 2008," he said, "we have reason to be confident that the situation will not be as severe as it is now, because we will be making efforts and expect to achieve results."