In dealing with South Korea's plan to upgrade its non-military nuclear program, the U.S. government continues to put a priority on a global nonproliferation regime, Washington officials said Thursday.

Thomas Countryman, assistant secretary of state for international security and nonproliferation, admitted that Seoul's pursuit of consent for enriching uranium and reprocessing plutonium is a major sticking point in bilateral talks on civilian nuclear cooperation.

"I'll simply reaffirm that with the ROK (South Korea), despite the fact that it is one of our very best allies, we continue to focus not on the political, not on the economic but on a consistent nonproliferation policy, what best strengthens the global nonproliferation regime. That remains our consistent guiding star," he said at a Senate hearing.

Countryman is the top U.S. negotiator in the talks with South Korea. The allies have struggled to rewrite the current accord, signed in 1974 and originally slated to expire in March this year.

In a bid to buy time, the two sides agreed to extend the negotiations for two years. A related bill passed both the Senate and the House earlier this week.

South Korea argues it should be allowed to enrich uranium and reprocess spent fuel to meet its growing energy needs at home and compete for the export of nuclear plants.

The U.S., however, is concerned about a possible negative impact to its global nonproliferation campaign. Deputy Secretary of Energy Daniel Poneman said Washington's policy has not changed.

"We have always sought to minimize enrichment and reprocessing," he said in the hearing before the Senate Foreign Relations Committee, led by Sen. Robert Menendez (D-NJ).

He was dismissing a view that the U.S. approach on the so-called 123 agreements with other nations might have changed, given a 2013 deal with Vietnam, which does not include a "gold standard" provision proscribing the partner nation from enriching uranium and reprocessing plutonium.

In signing an agreement with the U.S. in 2009 under the "gold standard," the United Arab Emirates (UAE) voluntarily renounced pursuing enrichment and reprocessing technologies and capabilities.

Poneman pointed out it was an exceptional case based on the UAE's voluntary decision, adding the U.S. policy remains unchanged and there is "no new flexibility."

Countryman also said the Vietnam agreement is not a model for a deal with South Korea.

"I don't quite agree that the Vietnam agreement has a precedential value for the ROK agreement," he said. "We're already far along in our discussions and on kind of a different plane of discussion."

Renewing the nuclear accord is one of the most sensitive issues between South Korea and the U.S., which have worked together for the denuclearization of North Korea.    The two sides had the ninth round of negotiations on the issue in early January. They plan to hold another round in April. (Yonhap)