SEOUL (AFP/Reuters) - Unfazed by international anger at its second nuclear bomb test, a defiant North Korea was said Saturday to be preparing to launch a long-range missile. The United States stressed it would not accept the North as a nuclear-armed state and warned that more atomic tests could spark an arms race in East Asia. A train carrying a long-range missile has been spotted at the weapons research centre near Pyongyang, South Koreas Yonhap news agency quoted an intelligence source as saying. The source said it may be a modified version of a Taepodong-2, which the North tested in 2006 and in April and which is theoretically capable of reaching Alaska. It usually takes about two months to set up a launch pad, but the process could be done in as little as two weeks, which means the North could launch a long-range missile as early as mid-June, the source said. Yonhap quoted a presidential official as saying the North may schedule a launch to coincide with a June 16 summit between South Korean President Lee Mying-Bak and US President Barack Obama in Washington. Two defence officials in Washington told AFP US satellite photos had shown vehicle activity at two launch sites, one in the west and one in the east. Diplomats at the United Nations Security Council are discussing a new resolution which could impose new sanctions to punish the North for Mondays nuclear test - its second since 2006. In a speech to the Asia Security Conference in Singapore, US Defence Secretary Robert Gates said the United States would not accept a nuclear-armed North Korea and sternly warned it against transferring any nuclear material. He said the threat from N Korea, which this week detonated a nuclear device and launched a series of missiles, could trigger an arms race in Asia. We will not stand idly by as North Korea builds the capability to wreak destruction on any target in the region or on us, Gates said. We will not accept North Korea as a nuclear state. He said the Obama Administration would hold North Korea fully accountable if it transferred any nuclear material outside its borders. Meanwhile, the first of 12 high-tech U.S. F-22 fighter jets landed on the southern Japanese island of Okinawa on Saturday, days after North Korea unnerved the region by detonating a nuclear device. The deployment underscores the US commitment to Japan as a vital regional partner and signals US resolve to ensure stability and security throughout the Pacific region, the US Air Force said in a statement on Friday. The aircraft, also known as Raptors, took off from their home base in Langley, Virginia earlier this week, and are set to be deployed in the region for four months. Okinawa plays host to the bulk of the roughly 50,000 U.S. military personnel based in Japan. Meanwhile, Japanese Defence Minister said his country would not initiate any hostilities against North Korea but is ready to defend itself. Defence Minister Yasukazu Hamada, speaking at a high-level security forum in Singapore, said Tokyo would be transparent with its military hardware purchases in order to prevent any misunderstanding with neighbours. We have mentioned that North Korea is a threat because of what has happened in the past but unless there are other countries moving to us, we will never start an action as such, Hamada told delegates at the annual Shangri-La Dialogue on security. In Japan, we have various and numerous constraints and internationally, we have also made clear that we do not use force in order to resolve conflict situations, he said. He told the forum Tokyo was looking at a range of weapon procurements including the F-22 fifth-generation fighter aircraft but stressed these were only for the defence of the country as such.