"We strongly oppose and reject the US and its vassal forces' arms buildup," North Korea'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.

The United States on Monday had 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 the formal name for South Korea.

The State Department approved the possible sale of the helicopters to South Korea, and the DSCA on Monday provided the required notification to the US Congress, which still needs to sign off on the transaction.

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 on Friday that US arms sales in the Pacific would prompt "the DPRK'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.

The Democratic People's Republic of Korea, or DPRK, is the formal name for North Korea.

The helicopter sale announcement came on 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.