A senior construction official urged cities building subways to be more careful and those planning subways not to rush.

"The current development scale and speed of China's rail transport is unprecedented," Huang Wei, vice-minister of construction, said at an international seminar for construction and safety of rail transport on Saturday.

Since the construction of the Beijing subway line 1 in 1965, 10 domestic cities had developed their own subway systems by the end of last year.

Currently, there are 21 lines in operation, with a total length of 581 km and capable of transporting 1.8 billion people a year, Huang said.

The coming 10 years, Huang said, will be a crucial time for the country's rail transport development.

Besides Beijing and Shanghai, which are still in the middle of a rapid expansion of their subway systems, another six cities - Shenyang, Harbin, Hangzhou, Suzhou, Chengdu and Xi'an, have gained approval to start their own subway projects.

Sixty-one lines with a total length of 1,700 km are under construction, Huang said.

"We expect to see a backbone subway system coming into being by then," he said.

"And with the fast urbanization, another 20 cities might be able to develop their own subways in 10 years," he said.

The relentless growth has spawned some potential hazards like the lack of experienced engineers rushing to meet deadlines irrespective of quality and safety measures.

"In addition, subways are top targets for human sabotage and even terrorists attacks, which means they require high safety standards," Huang said.

He urged cities to strengthen their quality control and safety measures and also map out emergency response plans.

Huang warned cities should not blindly pursue the building of subways.

"We should bear in mind the relationship between 'necessity' and 'possibility'," he said.

Construction of subways in cities should be in accordance with such factors as social development, economic affordability, environment and land supply, he said.

"We should promote the use of public transport, but not necessarily a subway, which costs billions and even more for maintenance," Huang said.

He also criticized some cities' unwise planning of subways which fail to achieve maximum social or economic gains.

"The subway is designed to relieve traffic pressure of the central areas first," he said, "however, the density of most of the planning of domestic subways at central areas is only half that of foreign countries, " Huang said.

Sui Zhenjiang, director of the Beijing municipal construction committee, said construction of another six subway lines in Beijing would start in the next few months.

Beijing has four rail lines with a total length of 114 km.