North Korea said it saw as "unjust" calls from global powers such as the United States for Pyongyang to verify claims it made in disarmament talks about producing arms-grade plutonium.

The North's KCNA news agency quoted an unnamed spokesman from its Foreign Ministry as also saying that South Korean-US military exercises, which started on Monday, had spoiled the atmosphere for the disarmament discussions.

"This situation compels the DPRK (North Korea) to heighten vigilance against such unjust demands as the 'verification in line with the international standard' recently claimed by the US as regards the nuclear issue," the spokesman said.

In late June, North Korea presented a long-delayed accounting of its atomic programme as part of an aid-for-denuclearisation deal it reached with five countries. The United States has said it will not reward Pyongyang for the disclosure until it can check the claims made in the atomic inventory.

North Korea, officially known as the Democratic People's Republic of Korea, routinely slams the annual South Korean-US military drills, calling them a prelude to invasion.

"The DPRK will increase its war deterrent in every way as long as the US and its followers continue posing military threats to it," the spokesman said.

The United States has said it will wait until a verification mechanism is in place before it considers taking North Korea off its terrorism blacklist. Once removed, the impoverished North will be better able to tap into international finance and trade.

The military exercises, called Ulchi Freedom Guardian, end on Friday. About 56,000 South Korean soldiers and 10,000 US troops are participating in the drills, which test computer, command and communication systems.

The United States has about 27,000 soldiers in South Korea to support the country's 670,000 troops. North Korea positions most of it 1.2 million troops near the border with the South.