The United States on Monday 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 proposed sale will 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," the Defense Security Cooperation Agency (DSCA) said in a statement.

The sale will improve "the security of a major ally that is a force for political stability and economic progress in the Indo-Pacific region," the statement said, using the US foreign policy term for the Asia Pacific region.

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

The principal contractors for the deal will be Boeing and Lockheed Martin, the DSCA said.

The announcement came on the same day that Washington and Seoul began major annual joint military drills, with new exercises aimed at containing the nuclear-armed North.

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