By Hyonhee Shin

SEOUL (Reuters) -South Korea has upgraded its U.S. alliance, President Yoon Suk Yeol said on Tuesday, as he briefed his cabinet on a trip to the United States which pledged more South Korean insight into nuclear planning in the event of war with North Korea.

Yoon held a summit with U.S. President Joe Biden in Washington last week as anxiety grows in South Korea over North Korea's rapidly evolving nuclear and missile capabilities and growing doubts that the U.S. would risk its security to help its Asian ally.

Calls have been growing in South Korea, even among some senior members of Yoon's party, for it to develop its own nuclear weapons. The U.S. opposes that, assuring South Korea of protection under the U.S. "nuclear umbrella".

"The alliance has a nuclear-based upgrade and has expanded to include supply chain, industrial and science and technology alliances," Yoon told his cabinet.

Yoon and Biden agreed in a "Washington Declaration" to launch a Nuclear Consultative Group to give South Korea more insight into, and a voice in, U.S. contingency planning to deter and respond to any nuclear incident in the region.

They also agreed on new partnerships on supply chains and science and technology, including cyber security, electric vehicles and batteries, quantum technology, foreign assistance and economic investment.Yoon said the nuclear planning mechanism would be more effective than NATO's Nuclear Planning Group by facilitating more frequent high-level bilateral discussions.

More U.S. "strategic assets" would be regularly deployed to South Korea to maintain an "overwhelming retaliatory posture", he said.

"We will build strong security, peace based on overwhelming strength, not the fake peace that relies on the good faith of the other party," Yoon said.

Some South Koreans, especially younger voters, have welcomed the summit's achievements, including Yoon's speech to the U.S. Congress and surprise singing at a White House state dinner.

But the opposition Democratic Party criticised a lack of progress on new U.S. rules on electric vehicle subsidies, which South Korea fears could hurt its automakers.

A Democratic Party spokesperson said Yoon "only sang a song at the world's most expensive karaoke" in return for billions of dollars of South Korean corporate investment.

Yoon's approval ratings slightly rose to 34.5%, a poll released on Monday by Realmeter showed, marking their first rebound in four weeks.

(Reporting by Hyonhee Shin; Editing by Gerry Doyle, Robert Birsel)