The United States and its allies should be able to determine within weeks whether North Korea's nuclear test involved a hydrogen bomb — as it claimed — or a far less powerful atomic bomb using a network of seismic and other devices that detect and measure nuclear explosions.

"They have pretty much surrounded North Korea," said Greg Thielmann, an analyst at the Arms Control Association and former State Department intelligence analyst. "My guess is within a couple weeks the U.S. will have reached some conclusions."

Experts will analyze both the size of the explosion and the radioactive particles emitted by the detonation, Thielmann and other analysts said.

H-bombs are more complex and difficult to make because they require fusion, or fusing of atoms, rather than fission, the splitting of atoms that creates the power of atomic bombs.

Experts are already skeptical that North Korea detonated an hydrogen bomb, despite claims from the capital Pyongyang.

Initial reports suggest the North Korean test yielded a six kiloton explosion, which would be significantly smaller than what a hydrogen bomb would typically produce, said Michaela Dodge, an analyst at Heritage Foundation, a Wasington-based think tank. The yield is more comparable to previous atomic bomb tests by North Korea.

A network of devices that measure seismic activity, called the International Monitoring System, have been established around the world as part of the international Comprehensive Test Ban Treaty.

The treaty organization has encouraged countries to allow monitors to be placed within their borders.

In addition, the United States has independent technical monitoring systems designed to measure such tests, including spy satellites and aircraft that can scoop up particles discharged by nuclear explosions.

"Much can be learned by the type of particles that are released," Thielman said. "That is what would definitely tell those countries monitoring what the bomb was made of."