(Corrects to remove references to South Korea and the United maintaining "pressure" on North Korea and to clarify that Song did not use the word "pressure")

By Scott Malone

CAMBRIDGE, Mass., Nov 5 (Reuters) - The United States and South Korea should ensure that North Korea sticks to its agreement to give up nuclear weapons, a process now at a crucial stage, South Korea's foreign minister said on Monday.

A team of U.S. technicians began disabling on Monday the reclusive communist nation's nuclear complex which produces weapons-grade plutonium, the U.S. State Department said.

North Korea last year defied international warnings and tested a nuclear device. However, in February it agreed to shut down its nuclear plant in exchange for fuel and other aid.

"We are now entering into the actual reverse process of nuclear proliferation," Song Min-soon, South Korea's Minister of Foreign Affairs and Trade, told Reuters in an interview prior to a speech at Harvard University in Cambridge, Massachusetts.

"We are actually now at the crucial stage, and that is the reason why I have to emphasize that we have to keep this momentum rolling," Song said in his speech.

He said the current steps involved cutting off and removing parts from North Korean nuclear facilities.

"(It) suits our common needs of convincing North Korea that negotiating away their nuclear program is good for the future of their country," Song said in the interview.

U.S. officials estimate the North has about 110 lbs (50 kg) of plutonium, which experts say could make six to eight bombs.

Under last February's agreement, the impoverished North will receive 1 million tonnes of heavy fuel oil or equivalent aid in exchange for shutting down its Yongbyon nuclear plant and admitting U.N. nuclear monitors.

The United States will also move toward removing North Korea from a U.S. terrorism blacklist.

Song said it was especially important to ensure Pyongyang sticks to its agreement during upcoming elections in South Korea and the United States.

South Koreans are due to vote for their next president in December, while in the United States the November 2008 presidential campaign is heating up. (Editing by Jason Szep)

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