North Korea (search), which insists it needs a nuclear deterrent against a U.S. invasion, threatened Monday to strengthen its "deterrent force" if the United States pursues policies the communist state deems hostile.

"If the United States more desperately pursues its hostile policy to isolate and stifle (North Korea) under the pretext of the 'nuclear issue' and 'human rights issue' ... the latter will react to it by further increasing its self-defensive deterrent force," an unnamed spokesman for the North Korea's Foreign Ministry said in a statement carried by the official Korean Central News Agency.

The spokesman condemned the North Korean Human Rights Act (search), a recent U.S. law aimed at improving human rights in the country. North Korea has repeatedly cited that law as an example of what it claims is Washington's hostile policy toward it.

"By nature the U.S. is the worst human rights graveyard in the world," the spokesman said. "This is clearly proved by what happened in Iraq."

Efforts are under way to persuade North Korea to return to six-party nuclear talks aimed at persuading Pyongyang (search) to give up its nuclear ambitions. However, the North has repeatedly insisted it won't return to the negotiating table until the United States abandons its "hostile" policy toward the country.

President Bush included North Korea in what he called the "axis of evil" along with prewar Iraq and Iran after the Sept. 11, 2001 attacks on the United States.

The two Koreas, China, Japan, Russia and the United States have held three rounds of talks on how to end North Korea's nuclear threat since last year, without any breakthroughs.

The planned fourth round of negotiations in September never happened because the North refused to attend.

North Korea insists on receiving economic aid and security guarantees in return for giving up its nuclear weapons ambitions, while the United States insists it immediately dismantle all nuclear facilities.

Some U.S. intelligence analysts say North Korea may have up to six nuclear weapons instead of the one or two the Central Intelligence Agency (search) estimates. North Korea says it has several plutonium-based nuclear weapons and denies U.S. allegations it has a secret uranium-based nuclear weapons program.

Some 33,000 U.S. soldiers are based in South Korea — a legacy of the 1950-53 Korean War, which ended without a peace treaty.