North Korea has added 20,000 soldiers to its feared special forces over the past two years and deployed an unspecified number of new battle tanks, South Korea's defence ministry said Thursday.

In its two-yearly white paper, the ministry said the size of the special forces had risen to 200,000 over the past two years.

But the total size of the North's military remained unchanged at about 1.19 million, the paper said.

"Threats from North Korea's asymmetric warfare capabilities such as special forces, artillery pieces and weapons of mass destruction have been rising steadily since 2008," Deputy Defence Minister Chang Kwang-Il told journalists.

Experts say the North increasingly focuses on unconventional or "asymmetric" weapons because its ageing conventional weapons are no match for South Korean or US equipment.

The report was released at a time of high cross-border tensions after the North last month shelled a South Korean border island, killing four people including civilians.

The paper said the North's frontline 170mm self-propelled artillery and 240mm multiple rocket launchers are capable of carrying out a "massive surprise bombardment" of Seoul and neighbouring areas, even if the overall number of artillery pieces changed little over the past two years.

It also confirmed the communist state deployed its new battle tank, "Pokpung-ho" ("Storm Tiger"), which the North claims is comparable or superior to the Russian T-72 developed in the 1990s.

The paper did not say how many of the new tanks the North has deployed for operational use. The overall number of its tanks rose to some 4,100 as of last month from 3,900 in 2008, the paper said.

But most of the newly added tanks had old-style equipment deployed in the rear, a military intelligence official told the briefing.

The North carried out its first atomic test in 2006 and a second in 2009.

It is suspected of having secured about 40 kg (88 pounds) of weapons-grade plutonium from its reactor at Yongbyon, the paper said, giving the same estimate as in 2008.

Estimates vary but the stockpile is thought to be enough to build six to eight nuclear weapons.

Concerns about the North's nuclear weapons programme deepened last month when it revealed a uranium enrichment plant -- a potential new source of bomb-making material -- to a visiting US scientist.