North Korea on Friday criticised recent US arms sales in Asia, calling a deal to deliver Apache helicopters to South Korea a "reckless, provocative act" and adding it would build up its own forces in response.

"We strongly oppose and reject the US and its vassal forces' arms buildup," Pyongyang's Foreign Ministry said in a statement, referring to the South.

"This is a reckless, provocative act of deliberately increasing the security instability in the region," it added, also criticising recent US sales to Japan and other Asia-Pacific allies of Washington.

On Monday, the United States announced its approval of a $3.5 billion sale to South Korea of up to 36 AH-64E Apache attack helicopters and related equipment, including missiles.

The US Defense Security Cooperation Agency (DSCA) said the sale would "improve the Republic of Korea's capability to meet current and future threats by providing a credible force capable of deterring adversaries and participating in regional operations", using South Korea's formal name.

While the State Department approved the sale, the US Congress must still sign off on the transaction.

Seoul's unification ministry said Friday that North Korea was the one "threatening peace and stability on the Korean Peninsula".

Pyongyang has defined inter-Korean relations as those of "two hostile nations" and is continuously "developing and advancing illegal nuclear and missiles", it added.

The South Korean government is "closely monitoring and thoroughly assessing North Korea's activities through close cooperation with relevant agencies," Kim In-ae, vice spokesperson for the ministry, told reporters.

- Ulchi Freedom Shield -

Washington is Seoul's key security ally and stations about 28,500 troops in South Korea, with their role including helping to protect it from its nuclear-armed neighbour.

Pyongyang also said Friday that US arms sales in the Pacific would prompt the North's "strategic deterrence" to be "further strengthened" in response.

"The prevailing situation urgently calls for thoroughly ensuring the military balance in the region by increasing the defence capabilities in every way in direct proportion to security challenges and threats that may result from the US arms sales," it said.

News of the helicopter sale came the same day that Washington and Seoul began major annual joint military drills, with new exercises aimed at containing the North.

The Ulchi Freedom Shield exercise runs until August 29 and will involve thousands of military personnel.

North Korea -- which attacked its neighbour in 1950, triggering the Korean War -- has always been infuriated by joint US-South Korean military exercises, decrying them as rehearsals for invasion.

Relations between the two Koreas are at one of their lowest points in years, with the North sending trash-carrying balloons across its southern border.