China’s military has conducted the first flight test of a new variant of one of its road-mobile intercontinental ballistic missiles in a sign that Beijing is increasing its strategic strike capability against the United States.

The test of a new DF-31B missile was conducted Sept. 25 from a missile test range in central China.

A Pentagon spokeswoman declined to provide details of the test.

"We continue to monitor China's military modernization, including its missile tests," Cynthia O. Smith, the spokeswoman, told the Washington Free Beacon.

No details of the missile test could be learned, but the test was believed to have been carried out from China’s Wuzhai test facility.

Nongovernment military analysts said the new missile likely is an increased-range or improved performance weapon, and possibly a multi-warhead version of the ICBM.

A Chinese military enthusiast website has identified the DF-31B as a mobile missile variant designed specifically for travel on rugged terrain or other difficult road conditions.

Mobile missiles are considered a greater strategic threat because tracking their location and targeting them in a conflict is very difficult. The missiles can be hidden in garages or caves to avoid detection by satellites and other sensors.

China has made clear in its state-run media that its nuclear forces are being developed for use against the United States. The Global Times reported Oct. 28 that a submarine-launched missile attack on the United States would kill between 5 million and 12 million Americans.

The new DF-31B is the latest addition to China’s rapidly growing nuclear missile arsenal that includes older silo-based missiles and five other road-mobile missiles. They include the long-range DF-31, DF-31A and DF-41 ICBMs, intermediate-range DF-26Cs, and medium-range DF-21s—a missile the Chinese have developed into a dual, nuclear-conventional weapon that includes an anti-ship variant. A DF-21 variant also is believed to be used as China’s anti-satellite missile system.

China has some 40 DF-31s and DF-31As, and the DF-41, which is expected to carry multiple-nuclear warheads, is said to be near deployment.

China also has deployed new submarine-launched missiles called the JL-2 that are based on new missile-firing nuclear-powered submarines that the U.S. Navy has said will begin their first sea patrols this year.

China also is working a high-technology hypersonic strike vehicle that is launched atop a missile and travels at extremely high speeds along the edge of the earth’s atmosphere. The glide vehicle is being designed to deliver a nuclear warhead through U.S. strategic missile defenses.

"They have an extraordinary selection of cruise missiles, and a ballistic missile force that they developed," Adm. Jonathan Greenert, chief of Naval Operations, told a security forum in August, adding that in a future conflict, China’s missiles pose the most serious threat.

"If [a conflict is] in their backyard, I’m a little worried about their ballistic missile [force] because of its reach," Greenert said.

China in late July conducted a flight test of a DF-31A, the fourth known flight test of that new missile in the past two years.

The latest missile test, which was not announced by the Chinese government, highlights Beijing’s largely secret strategic nuclear forces buildup.

Rick Fisher, an analyst who closely monitors the Chinese military, said the testing of a new DF-31 variant should be a worry for U.S. security officials.

"The emergence of a third version of the DF-31 raises the question of whether there is a multiple warhead version," said Fisher, with the International Assessment and Strategy Center.

Fisher said the DF-31B also might be a silo-based missile or one designed specifically for China’s so-called "Great Underground Wall"—a network of 3,000 miles of tunnels and underground nuclear facilities that was first revealed several years ago.

"China has a track record of using warhead systems on multiple missile programs," he said. "The advent of multiple warheads on the DF-41 may indicate new versions of the DF-31 may be so equipped. If real, this would accelerate China's warhead growth."

The testing of a third DF-31 variant, along with Moscow’s nuclear weapons modernization program "places greater pressure on Washington to proceed with modernizing America's nuclear deterrent," Fisher said.

Mark Stokes, a specialist on Chinese strategic forces, said the new ICBM variant could be a technically improved weapon.

"A DF-31B would most likely be an incremental, phased improvement on the DF-31A," said Stokes, with the Project 2049 Institute.

"As a matter of PLA defense industrial process, R&D on an improved variant would have begun after the DF-31A entered full rate production," Stokes said.

The Pentagon’s most recent annual report on the Chinese military states "The Second Artillery continues to modernize its nuclear forces by enhancing its silo-based intercontinental ballistic missiles (ICBMs) and adding more survivable mobile delivery systems."

"In recent years, the road-mobile, solid-propellant [DF-31A] ICBM has entered service," the report said, adding that "China also is developing a new road-mobile ICBM known as the Dong Feng-41 (DF-41), possibly capable of carrying multiple independently targetable re-entry vehicles (MIRV)."