Energy-starved North Korea has recently increased military manoeuvres, arousing suspicions it may have diverted fuel oil provided under a multinational aid-for-disarmament deal, a report said Sunday.

"It is noteworthy that the North's armoured units have sharply increased exercises in the ongoing winter manoeuvring," an unnamed military source told South Korea's Yonhap news agency, adding Seoul and Washington were analysing the moves.

The new drills involved fighter jets and armoured vehicles in contrast to previous years which had focused on artillery rather than such oil-consuming exercises, the source added.

"We understand North Korea has been enhancing the number of flights flown for training," another unnamed military source told Yonhap.

The sources attributed the increased manoeuvring of armored vehicles and aircraft to the North's "improving" oil supply conditions, without elaborating.

South Korea's defence ministry refused to comment on the report.

But Yonhap said US and South Korean intelligence experts were trying to verify whether the North had diverted the provided fuel oil or earnings from inter-Korean business for military purposes.

Under a six-nation agreement last year, the North would receive one million tons of heavy fuel oil or equivalent aid in return for disabling its nuclear facilities and declaring all relevant programmes.

The impoverished North has also expanded cooperation with South Korean businesses to help earn hard currency.

South Korean negotiators of the disarmament deal have privately said the fuel oil is unsuitable for armoured vehicles, but Yonhap said intelligence authorities were wondering whether it was refined for military purposes.

North Korea's winter military exercises came as South Korean and US troops were preparing to launch a joint drill codenamed "Key Resolve" between March 2 and 7 here.

The North last week condemned the upcoming US-South Korean exercise as preparation for an invasion of the communist state, allegations which are routinely and flatly denied by Seoul and Washington.

The exercise is the first to test Seoul's ability to wage war under a scenario in which South Korea has regained wartime control of its troops from the US. The shift in control is due in 2012.

Some 28,000 US troops are stationed here to back up South Korea's 680,000-strong forces against any threat from the North's 1.1 million-member military.