SEOUL, August 20, 2008 (AFP) - North Korea Wednesday stepped up its criticism of a major US-South Korean military exercise, saying it could impede progress on an international nuclear disarmament deal.

The annual exercise involving tens of thousands of troops began Monday. Seoul and Washington say it is purely defensive while Pyongyang says it is aimed at starting a new war on the peninsula.

'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,' said a foreign ministry spokesman.

'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 North will 'actively take corresponding practical measures,' the spokesman said as quoted by the official Korean Central News Agency, describing the exercise as a clear proof of Washington's hostile policy.

The US and North Korea cannot agree on ways to verify the nuclear declaration which the North submitted in June as part of a six-nation disarmament deal.

The US refuses to remove the North from a terrorism blacklist until the issue is solved.

On Monday the North criticised the US for failing to act, saying Washington was not honouring its side of the deal.