Government officials and analysts in Seoul say the ball is now in the U.S. court. On Monday, Pyongyang increased pressure on the United States, calling for a peaceful resolution of the crisis.
Last week, Pyongyang allowed U.S. congressional aides and nuclear experts to visit its main nuclear complex in Yongbyon.
"As everybody knows, the United States compelled the DPRK (North Korea) to build nuclear deterrent," the North Korean Foreign Ministry said in a statement carried by the official Korean Central News Agency. "We showed this to Lewis and his party this time."
South Korean officials said the U.S. delegates, including John Lewis, a professor emeritus at Stanford University, made a one-day visit to the Yongbyon complex during their a five-day trip to North Korea that ended Saturday, which marked the first anniversary of Pyongyang's withdrawal from the nuclear non-proliferation treaty. The Americans were the first outsiders to visit the facility, 50 miles north of Pyongyang, since U.N. inspectors were expelled in late 2002.
It is unclear if North's use of the term "nuclear deterrent" meant atomic bombs, but many analysts in Seoul agree North Korea is on the verge of possessing nuclear weapons. The CIA believes the Stalinist state may already possess one or two nuclear weapons.
Seoul's intelligence officials believe North Korea has recently restarted a 5-megawatt reactor at the nuclear complex, which had been frozen under a 1994 accord.
"The next step North Korea is expected to take is conducting a nuclear test or declaring itself as a nuclear-armed state," said Cheon Seong-whun, a researcher at the Korean Institute for National Unification, a government-run think tank. "The United States, South Korea and other allies should consider Pyongyang's atomic arms as a fait accompli and step up cooperation to deter the North from producing further nuclear weapons and exporting them."
The North Koreans said the visit was designed "to give the Americans an opportunity to confirm the reality by themselves and ensure transparency, as speculative reports and ambiguous information about the DPRK's nuclear activities are throwing hurdles in the way of settling the nuclear issue."
Analysts in Seoul said the U.S. group appeared to have seen reprocessed plutonium, an ingredient for nuclear bombs, though the American warned against "premature" conclusions.
North Korea has said it has completed reprocessing 8,000 spent nuclear fuel rods in a process that could yield weapons-grade plutonium, and has vowed to demonstrate its nuclear deterrent at "an appropriate time."
Experts say if the 8,000 fuel rods were efficiently converted, they could produce 30-35 kilograms of weapons-grade plutonium to make up to seven atomic bombs.
Analysts say the North's opening of the Yongbyun complex to the U.S. team was aimed at pressing the Bush administration to start bargaining seriously. In the past, North Korea has skillfully employed a "strategy of ambiguity" over its nuclear program, never openly declaring itself a nuclear power.
"This proves that its nuclear threat is no more blackmail," KINU's Cheon said.
The United States will now be under pressure over whether to continue diplomatic efforts or impose naval or aerial blockades to keep the North from exporting atomic arms, he said.
Koh Yu-hwan, a North Korea specialist at Dongguk University, says the North's comments about nuclear deterrence reflect Pyongyang's desire to speed up negotiations with Washington.
"As the United States has rebuffed Pyongyang's repeated nuclear claims, Pyongyang might have displayed to the U.S. visitors its nuclear reprocessing facilities and plutonium," he said.
In an indication that Pyongyang was eager to engage Washington, North Korea Monday offered to freeze its nuclear reactors producing weapons grade plutonium if compensated by the United States. It was the North's third offer to freeze its nuclear program in less than three weeks.