China recently upgraded its subway system in Beijing and revealed that its mass transit was hardened to withstand nuclear blasts or chemical gas attacks in a future war, state-run media reported last month.

The disclosure of the military aspects of the underground rail system followed completion and opening of a new subway line in the Chinese capital Dec. 30, along with the extension of several other lines. The subway upgrade is part of an effort to ease gridlocked traffic in the city of 20 million people.

According to Chinese civil defense officials quoted Dec. 5 in the Global Times, a newspaper published by the Chinese Communist Party Central Committee, the subway can "withstand a nuclear or poison gas attack."

A U.S. official said the disclosure of the subway’s capabilities to withstand attack is unusual since it highlights Beijing’s strategic nuclear modernization program, something normally kept secret from state-controlled media. The strategic nuclear buildup includes the expansion of offensive nuclear forces, missile defenses, and anti-satellite arms.

China is building new long-range mobile missiles, including the DF-41, and plans to deploy up to eight new ballistic missile submarines. Reports from Asia indicate the Chinese military is also planning to build new long-range strategic nuclear bombers.

Russia too is expanding its nuclear forces with new submarines and missiles. Moscow announced last year that it is also constructing some 5,000 underground bomb shelters in Russia’s capital in anticipation of a possible future nuclear conflict.

By contrast, the U.S. government has done little to bolster civil defense measures, preferring the largely outdated concept of mutual assured destruction that leaves populations vulnerable to attack and building only limited missile defenses that the Obama administration has said are not designed to counter Chinese or Russian nuclear strikes.

The Obama administration instead is seeking deep cuts in U.S. nuclear forces as part of President Barack Obama’s policy of seeking the elimination of all nuclear arms.

According to the Global Times report, the new subway lines were "designed to be used in the event of an emergency, for underground evacuation from one station to another, emergency shelter, and storage for emergency supplies."

A military engineer identified only as Hu and as part of the Chinese military’s Second Artillery Corps, which builds and deploys China’s nuclear arsenal, helped design the civil defense aspects of the subway.

Special steel-reinforced gates installed on all subway tunnels and used to separate stations are one key feature of the reinforced subway. Hu said it is designed to protect people who seek shelter during a heavy storm, toxic gas attack, or a nuclear strike.

"The station has three hours of breathable air after the gates are closed, isolating the station from the outside world," Hu was quoted as saying.

"Although each gate weighs around 7 tons, it takes just three minutes for two adults to open or close it manually," she said.

The new blast gates were introduced into subway construction projects in 2007.

A second Chinese official, identified in the report as Liang, said each subway also has an air filtration system in case of a chemical weapons gas attack. The system is designed to keep air flowing into the station.

"People can actually shelter in the subway for more than three hours because of this system," Liang said.

Above-ground subway exits also can be sealed during an attack, Liang said, using heavy blast doors concealed behind temporary walls.

Additional civil defense barriers and doors are being installed in the Beijing subway later, according to Cao Yanping, deputy director of the Beijing Municipal Civil Air-Raid Shelter.

Jiang Hao, a Chinese military engineer from the 4th Engineer Design & Research Institute of General Staff Department, told the newspaper that blast gates already are in use in cities such as Nanjing, in Jiangsu Province, and Shenyang, in Liaoning Province.

"The new facilities also have other defensive capabilities like emergency communication equipment at each station, which makes effective communication possible during a conflict," Jiang Hao, the engineer, told reporters in Beijing.

China’s network of underground tunnels for nuclear weapons and missiles was disclosed only recently, and highlighted in Georgetown University’s Asian Arms Control Project, dubbed it China’s Underground Great Wall.

The Pentagon first disclosed the nuclear tunnel complex stretching an estimated 3,000 miles in its annual report to Congress on the Chinese military in 2011.

"China’s strategic missile force, the Second Artillery Corps (SAC), has developed and utilized [underground facilities] since deploying its oldest liquid-fueled missile systems and continues to utilize them to protect and conceal their newest and most modern solid-fueled mobile missiles," the report stated.

The facilities are used for storing and hiding missiles and nuclear warheads, and for command bunkers hardened against nuclear attacks.

China has been tunneling and hiding its nuclear forces since the early 1950s but the first public disclosure of the effort came in 2010 during the anniversary of the Second Artillery Corps.

Until then, both Beijing and the Pentagon kept most details of Chinese underground nuclear facilities and arms secret.