The planned US deployment of a THAAD anti-missile system in South Korea is not negotiable as part of efforts to agree new United Nations sanctions on North Korea after its latest nuclear test, a senior US official said on Friday.

China, whose full backing is widely seen as crucial for sanctions on North Korea to be effective, is strongly opposed to the deployment of the Terminal High Altitude Area Defense system and some experts have argued it should be part of talks on new U.N. measures.

But asked whether THAAD was negotiable, Daniel Russel, the senior US diplomat for East Asia, referred to an agreement by the United States and South Korea on the deployment.

"No. The two countries have made a decision," he told Reuters.

Discussions are already under way on a possible new U.N. sanctions resolution on North Korea after it conducted its fifth and largest nuclear test on Sept. 9. Analysts and diplomats say much depends on China's attitude.

South Korea's Foreign Minister Yun Byung-se on Thursday told the U.N. General Assembly that North Korea's nuclear and missile programs presented "a direct existential threat" and said his country had no choice but to take "necessary defensive measures."

Two experts from the Massachusetts Institute of Technology's Center for International Studies last week argued that if China were to agree to serious graduated sanctions on North Korea, the United States could agree to freeze the number of ground-based missile interceptors on the Korean Peninsula.

In an article in The National Interest magazine, Eric Heginbotham and Richard Samuels said that as part of a set of incentives to China, Washington "might also agree, after consulting South Korea, to withdraw THAAD from the peninsula when North Korean nuclear weapons no longer pose a threat."

China is North Korea's main ally, but has been angered by its repeated missile and nuclear tests and backed tough U.N. sanctions on Pyongyang in March.

Beijing has also said it will work within the United Nations to formulate a necessary response to the latest nuclear test, but questions remain as to whether it is willing agree on tough enough steps to force North Korea to abandon its nuclear weapons program.

Publicly, China has not linked the THAAD deployment with whether it will support such sanctions, but it says the system compromises its own security and that North Korea's recent belligerence is due to the deployment plan.

However, last month the U.N. Security Council was unable to condemn the launch of a missile by North Korea that landed near Japan because China wanted the statement to oppose the planned deployment of a US anti-missile defense system in South Korea.

Russia is also opposed to THAAD deployment and on Friday its foreign minister, Sergei Lavrov, told the United Nations General Assembly it was wrong of the United States to use the threat of an attack by North Korea to deploy the system.

"It is inadmissible to use this situation as a pretext for massive militarization of northeast Asia and deployment of another position area for US global missile defense there," he said.