SEOUL (Reuters) - North Korea said on Friday it was closer to a second way of making nuclear weapons, a move analysts saw as a new tactic to put pressure on the international community after a month of conciliatory gestures. The United States vowed to keep up its tough sanctions regime on North Korea Friday after Pyongyang said it had reached the final stage of enriching uranium. We continue to be committed to ensuring that North Korea upholds its international obligations and we continue to strongly implement the sanctions that were approved, White House spokesman Robert Gibbs said. The US also said it is very concerned by North Koreas claims. Meanwhile, Russia described the announcement as very alarming. Bosworth was in Beijing on a trip to Asia to discuss ways to bring Pyongyang back to long-stalled negotiations on giving up its nuclear ambitions. Experimental uranium enrichment has successfully been conducted to enter into completion phase, the Norths KCNA news agency quoted its United Nations delegation as saying in a letter to the head of the UN Security Council. The US has long suspected DPRK of having a secret programme to enrich uranium for weapons. Experts have said it has not developed anything near a full-scale uranium programme while it has enough plutonium for six to 8 bombs. I think for all of us, it reconfirms the necessity to maintain a coordinated position on the need for complete, verifiable denuclearisation of the Korean peninsula, Bosworth said, after meetings with Chinese officials. The North said its latest steps were in response to tighter sanctions. These reports cannot but provoke concern, Russias Interfax news agency quoted an unidentified Foreign Ministry official as saying A very alarming precedent is being created by such an open and demonstrative disdain for resolutions of the United Nations Security Council. At the United Nations, the mission of the United States, which currently holds the Security Council presidency, said it had received the North Korean letter and circulated it to the other 14 members of the council. UN sanctions have hurt the impoverished Norths arms trade, one of its few significant exports, and analysts said it may be angered its latest attempts at conciliation with the outside world have been largely rebuffed. The United States has refused to ease up on the sanctions, and sent its point man for their enforcement to Asia last month to build support. Now they (North Korea) are taking the road that they know will drive a response out of all countries the military way and leaving them to decide what to do, said Cho Myung-chul, an expert on the North at the Korea Institute for International Economic Policy. North Korea added that reprocessing of spent nuclear fuel rods was at its final phase and extracted plutonium was being weaponised. We are prepared for both dialogue and sanctions. If some permanent members of the UNSC wish to put sanctions first before dialogue, we would respond with bolstering our nuclear deterrence first before we meet them in a dialogue. In another move that serves as a reminder of the tension on the divided peninsula, a North Korean patrol vessel crossed briefly into the Souths waters on Friday but returned without incident, the Souths Joint Chiefs of Staff said. They are taking a mixed strategy, which I would call the sweet and sour approach. This keeps their adversaries guessing and it makes it more difficult to formulate policy, said Peter Beck, research fellow at Stanford University and a specialist on Korean affairs. Market players, used to North Korea blowing hot and cold, said the latest rumblings had little impact on early trading. Meanwhile, South Korea Friday pledged a stern response to North Korean threats and provocations. We find it very regrettable that the Norths attitude goes against the UN resolutions, the foreign ministry said in a statement. In a letter to the Security Council, the North said earlier Friday that an experiment to enrich uranium a second route to a nuclear bomb has entered its final phase. It also said it was building more atomic bombs from spent reactor fuel rods. This is intolerable, the ministry said. The government will take a stern and consistent response to North Koreas threats and provocations. Spokesman Moon Tae-Young said South Koreans will have in-depth talks with US special envoy on North Korea Steven Bosworth, who arrives here later Friday from Beijing. Bosworth is visiting the region for talks aimed at restarting six-party nuclear disarmament talks, which the North is boycotting. South Koreas nuclear envoy Wi Sung-Lac said the Norths latest statement is not helpful and he would discuss it with Bosworth.