WASHINGTON (CNN) -- U.S. and North Korean negotiators made progress in nearly a week of talks, but they did not reach an agreement to inspect a suspected North Korean nuclear weapons site, the U.S. State Department said.

The two sides concluded six days of discussions in Washington and New York Friday afternoon, with North Koreans still refusing access to an underground complex at Kumchang-ri, which the United States believes is part of a nuclear weapons program.

"Although gaps remain, the parties approached the issues in a problem-solving manner," State Department Deputy Spokesman James Foley said in a statement. "The two sides recognized that progress was made and agreed to meet as soon as possible at a venue and date to be determined."

The United States has said North Korea's continued denial of access could threaten a 1994 deal, when the U.S. agreed to supply North Korea with heavy fuel oil and light water nuclear reactors in exchange for a North Korean freeze on its nuclear program.

The two sides began negotiations on the suspected site in August. Last month, North Korea offered to let inspectors into the complex if the United States would pay $300 million if no evidence of nuclear activities was found. The United States refused.

The State Department said the latest round of talks was conducted in a "serious atmosphere." But at times, the North Koreans angrily denounced U.S. policies.

The U.S. is concerned with North Korea's developing ballistic missile program

On Tuesday, North Korean Army General O Kum-chol vowed his country's 1.2 million troops were ready to defeat the United States and its allies.

"Soldiers of the Korean People's Army are firmly determined to annihilate the U.S. imperialists, Japanese reactionaries and South Korean puppets at one stroke," he said. "Our People's Army will let loose its pent-up anger and shower thunderbolts of revenge upon the enemies like an angry tiger."

In addition to the suspected nuclear site, the United States is concerned with North Korea's development of a ballistic missile program and its exports of military hardware to other countries.

A U.S. official said the United States was watching construction elsewhere in North Korea to see if Pyongyang was building new missile launch bases.

Reuters contributed to this report.