North Korea is fitting U.S.-made machine guns to its large fleet of border patrol boats, leaving Washington scrambling to work out how it obtained them, South Korea's Yonhap news agency said Tuesday, citing unnamed military sources.

It said the cannons are thought to be 12.7 mm Gatling guns made by General Electric. The report added that U.S. and South Korean sources were trying to track down how the weapons ended up in North Korea.

A contemporary version of the gun, known as the GAU-19 and manufactured by General Dynamics, is reportedly in use by the armed forces of Mexico, Colombia and Oman. The cannon, which uses NATO-standard 50-caliber ammunition, is also reportedly used by the Japan Coast Guard.

The three-barrel weapon offers "devastating" firepower, General Dynamics says on its website.

"The GAU-19 is ideal for use on a variety of surface navy ships and patrol boats," it says.

"Firing at 1,300 shots per minute, the GAU-19 provides unparalleled firepower to counter both air attacks and high-speed surface threats."

Yonhap quoted a military insider as saying the upgrade could pose a serious challenge to South Korean forces.

"North Korea has been seen replacing its aging weapon systems with the rotary gun system," the source said. "If all the 380-something North Korean littoral combat boats are armed with the machine gun, this would pose an added threat to our Navy."

Separately, Yonhap said the North is also moving to fit radar-evading stealth technology to its littoral combat vessels.