North Korea vowed Friday to bolster its atomic arsenal in response to what it called Washington's "persistent hostile policy," even as a special envoy for President Barack Obama traveled to the region in a bid to draw Pyongyang back to nuclear negotiations. Washington is ready and willing to talk directly with Pyongyang, envoy Stephen Bosworth said in Beijing before heading Friday to Seoul. "The United States reiterates its desire to engage both multilaterally and bilaterally with North Korea," Bosworth told reporters after holding talks with senior Chinese officials Thursday. "We believe very strongly that the solution to the tensions and problems of the area now lies in dialogue and negotiation." But North Korea dismissed the Obama administration's stance as "unchanged" from a previous policy of hostility. "Nothing would be expected from the US, which remains unchanged in its hostility toward its dialogue partner," the North's Foreign Ministry said in a statement carried Friday by state media. The North "will bolster its nuclear deterrent as it has already clarified." Pyongyang, which carried out a nuclear test in 2006 and is believed to have enough plutonium to make at least a half-dozen atomic bombs, has been locked in a standoff over its nuclear program for months. The regime agreed in 2007 to begin dismantling its nuclear program in exchange for 1 million tons of fuel oil and other concessions. Disablement began later that year, with Pyongyang blowing up the cooling tower at its main nuclear facility in June 2008 in a dramatic show of its commitment to denuclearization. But disablement came to a halt a month later as Pyongyang wrangled with Washington over how to verify its past atomic activities. The latest round of talks in December failed to push the process forward. North Korea's plans to launch a rocket last month heightened tensions with its nuclear partners. Pyongyang called the April 5 launch a successful bid to send a satellite into space. The US and others saw it as a violation of UN Security Council resolutions barring the North from ballistic missile-related activity since the same technology can be used to fire an intercontinental missile mounted with nuclear arms. The UN Security Council condemned the launch and punished the regime by slapping sanctions on three North Korean firms. North Korea retaliated by quitting the nuclear negotiations, kicking out US and UN inspectors and warning it may conduct nuclear or long-range missile tests if the UN and Washington refuse to apologize for the censure. South Korea's Chosun Ilbo newspaper recently reported "brisk" activity has been detected at North Korea's nuclear test site, citing an unnamed South Korean government source. The report could not be confirmed. In Washington, a counterproliferation official would not confirm whether the United States believes North Korea is on the cusp of conducting a nuclear test but said Pyongyang is capable of conducting one quickly if desired. The official spoke Thursday on condition of anonymity. Arriving in Seoul, Bosworth refused to respond to Pyongyang's latest threat, saying: "I am not going to react to every statement coming out of North Korea. I am here to have talks with the South Korean government." Bosworth and nuclear talks envoy Sung Kim had no set plans to visit Pyongyang during his regional tour, which includes stops in Tokyo and Moscow in addition to Beijing and Seoul, the State Department said. While campaigning for the presidency, Obama said he would be willing to meet with North Korean leader Kim Jong Il if it helps denuclearization. His administration has so far maintained its support for talks within the framework of nuclear negotiations. Secretary of State Hillary Rodham Clinton said Washington's goal is to get North Korea back to the disarmament process, but warned it would be difficult. "We may have to show some patience before that is achieved," she told reporters after talks Thursday with Russian Foreign Minister Sergey Lavrov, whose trip to Pyongyang last month produced little progress in getting the North back to the negotiating table.