The State Department said it had given its approval for South Korea to spend $3 billion on 24 "Romeo" Seahawk naval helicopters, used to track and destroy submarines as well as land targets.

The State Department also approved $1.2 billion in sales for upgrades in South Korea's Apaches, a backbone attack helicopter.

The sale "will improve Republic of Korea's capability to meet current and future threats" and will serve as a "credible force that is capable of deterring adversaries," a State Department statement said.

South Korean President Lee Jae Myung promised during a summit in November with counterpart Donald Trump that Seoul would buy $25 billion in US military equipment by 2030 as well as pay $33 billion to support American forces in South Korea.

Trump has persistently pressed allies to spend more on defense and questioned the need for US forces overseas.

Lee, a dove, has advocated talks with North Korea without preconditions but has found no reciprocation from Pyongyang.

In his first term, Trump met three times with North Korean leader Kim Jong Un, even saying they "fell in love," but has spent little time on Pyongyang in his latest term.