America has warned it "will not accept" a nuclear-armed North Korea as new intelligence data showed that the secretive state was preparing a fresh missile launch, which could take place in two weeks' time.

The fresh developments increased the tension yet further yesterday in a game of diplomatic brinkmanship that seems destined to put Washington and Pyongyang on a collision course.

The US defence secretary, Robert Gates, used the opportunity of a security conference in Singapore to force home America's view that North Korea's recent nuclear test and missile launches were seen as a direct threat. "We will not stand idly by as North Korea builds the capability to wreak destruction on any target in the region or on us... we will not accept North Korea as a nuclear state," he said.

North Korea has test-fired six missiles since detonating a nuclear device on Monday, with the latest being a short-range missile launch on Friday. In a statement, North Korea denounced its critics as "hypocrites" and warned of "adopting self-defensive countermeasures" if action were taken against it at the UN.

According to the South Korean news agency Yonhap, spy satellites have captured images that show the country is busy preparing more tests. The images appear to show an intercontinental ballistic missile being moved by train from the capital to a launch pad at Musudan-ri in the north-east of the country. The missile is thought to be a Taepodong-2, which is in theory capable of hitting US soil. The missile could be launched within two weeks. Other images show vehicle activity at test sites and Chinese fishing boats fleeing the area.

South Korean and US forces in the region have been placed on the second-highest alert level. The last time the joint forces raised the "Watchcon" surveillance alert was after a 2006 North Korean test that proved the regime was a nuclear power. North Korea has said that it no longer considers itself bound by the armistice that ended the Korean war in 1953 and ushered in five decades of uneasy peace on the peninsula.

There is huge speculation about what North Korea seeks to gain by raising the tension levels. The impoverished country may be using its nuclear programme as a bargaining tool to obtain aid and ease sanctions. But there is also the possibility that recent developments are linked to the country's murky internal politics. Its leader, Kim Jong-il, is believed to be in failing health and there is no clear line of succession.

Washington does not fear a direct attack by North Korea, but is concerned that the country will export its nuclear technology to other rogue states or terrorist networks. The country, crippled by sanctions and its own disastrous economy, has already acquired billions of dollars by secretly exporting missile technology to the Middle East and Pakistan. In his speech in Singapore, Gates warned that America would hold Pyongyang "fully accountable" for the proliferation of any nuclear material or technology. "The transfer of nuclear weapons or material by North Korea to states or non-state entities would be considered a grave threat to the US and our allies. And we would hold North Korea fully accountable," Gates said.

However, co-ordinating an effective international diplomatic response to North Korea has not been easy. Russia and China have stymied efforts to isolate the country at the UN, preventing a united front to condemn the country's nuclear ambitions.