South Korea on Tuesday rejected Pyongyang's proposal for a joint investigation into three crashed drones that Seoul believes originated in North Korea.

"The proposal is not worth considering," defence ministry spokesman Kim Min-Seok told reporters.

He accused North Korea, which has flatly denied having anything to do with the drones, of distorting basic facts "to cover up its provocations".

The ministry said last week it was convinced that three rudimentary unmanned aerial vehicles (UAV) found in recent weeks in three different places, including a front-line island, were of North Korean origin.

Pyongyang accused Seoul of fabricating a link in order to smear North Korea, and called for a joint probe.

"It is an inveterate bad habit of the South Korean authorities to cook up shocking cases," an inspection group of the North's powerful National Defence Commission said in a statement Monday.

The statement cited the case of the South Korean naval corvette, Cheonan, which sank with the loss of 46 lives in 2010 near the disputed Yellow Sea border.

An investigation by a South Korean-led international commission concluded the vessel had been sunk by a torpedo from a North Korean submarine -- a charge Pyongyang has always angrily denied.

A team of US experts arrived in Seoul on Monday to help with further analysis of the three retrieved UAVs and the cameras they were carrying.

Their memory chips contained pictures of border areas and the capital Seoul, including the presidential palace, although they were of low quality, the defence ministry said.