Falling house prices and strict supervision by authorities contributed to the drop in illegal land use cases in the first quarter of the year, experts said.

There were 9,462 illegal land use cases from January to March, inspectors found. The number was down by 3.7 percent year-on-year.

The total area of land being used illegally amounted to 3,200 hectares, a 35 percent drop on a yearly basis, Wang Ling, deputy director of the law enforcement and supervision bureau under the Ministry of Land and Resources, said on Wednesday at a news conference.

"The slump in property prices is a factor to be reckoned with," said Yan Jinming, a land management professor at the Beijing-based Renmin University of China.

He said tighter policies had reduced developers' profits, so developers were not investing in as much land. Consequently, illegal land use had been decreasing to some degree.

"But housing prices are still not at the right level. Further falls may contribute more to this issue," said Dang Guoying, a researcher with the Rural Development Institute of the Chinese Academy of Social Sciences.

There has been a quarterly slump in residential land prices in 48 cities, such as Beijing and Wenzhou, in Zhejiang province, this year, and prices were flat in another 10 cities, according to ministry figures released on Tuesday.

"There is room for a bigger housing price decline, due to the drop in land prices, which accounts for a large part of the investment in the real estate market," said Dang.

Strict and regular supervision is also an important reason for the drop in illegal land use, said a researcher affiliated with the ministry, who declined to be named.

Figures released by the ministry on Wednesday showed that in the first quarter, 3,864 cases had been put on the record for investigation, an increase of 2.9 percent year-on-year.

"The ministry and local departments have adopted stricter measures to control the rise of illegal land use by, for example, talking with officials in charge of these issues and setting up an accountability system," the researcher said. "Their performance will affect their promotion."

But the quarterly report on illegal land use also warns that more construction projects may have begun or continued their construction since April, which may mean an increase in illegal land-use cases. Thus, the supervision sector needs to pay more attention, the ministry said.

Dang said enforcement by the ministry and local departments is far from enough to provide a radical cure for illegal land use. A more efficient and effective way would be to make the land-planning documents have the force of law. People who break the law should receive harsh punishment, he added.