North Korea could test-fire a long-range missile designed to strike United States territory in June and may also be gearing up for skirmishes with the South around their disputed sea border, according to South Korean news reports on Monday.
The reports cite South Korean intelligence sources as saying the tests involve an intercontinental ballistic missile with a range of up to 6,500km.
North Korea last week carried out a nuclear test, short-range missile tests and threatened to attack the South. It also warned of further measures if the United Nations tries to punish it.
North Korea plans to hold a trial this week for two American journalists it took into custody along its border with China several months ago after charging them with "hostile acts."
In April, North Korea fired a rocket from its east coast Musudan-ri missile range, which was widely seen as a disguised test of its long-range Taepodong-2 missile that violated UN resolutions banning it from ballistic missile launches.
The rocket splashed down in the Pacific Ocean about 3,000km after launch, experts said, indicating the North needed further testing to perfect its intercontinental ballistic missile technology.
The Alaskan coast is about 4,800km from North Korea while the US West Coast is at least 8,000km away.
North Korea has threatened to test-fire an intercontinental ballistic if the UN Security Council does not apologise for tightening sanctions after the April rocket launch.
Western diplomats said Russia and China have agreed in principle that North Korea should be sanctioned for its nuclear test, but it was not clear what kind of penalties they would support. Both are generally reluctant to approve sanctions.
Meanwhile, North Korea has declared a wide area of Yellow Sea waters off its west coast off limits until the end of July because it may be preparing for a clash with the South, a report in South Korea's biggest newspaper, the Chosun Ilbo, said.
The two Koreas fought deadly battles in 1999 and 2002 near a Yellow Sea border called the Northern Limit Line, set by US-led UN forces at the end of the Korean War. The North has said the border is invalid and last week warned ships from the South it could not guarantee their safety if they sailed in that area.
Nuclear-armed North 'unacceptable'
The United States and Japan agreed on Monday that they cannot accept a nuclear-armed North Korea, saying they are deeply concerned about recent "destabilising" actions by the isolated communist state.
"We absolutely cannot accept that North Korea will have nuclear weapons," Japanese Vice-Foreign Minister Mitoji Yabunaka, said after meeting US Deputy Secretary of State Jim Steinberg.
Mr Steinberg said relevant countries have been holding discussions on what steps they can take at the UN Security Council and elsewhere to make clear to Pyongyang that its recent actions are "a bad path to go down".
"We stand united in making clear our deep concerns about North Korea's highly destabilising actions," Mr Steinberg said in Tokyo. "We reaffirm our commitment to work together along with South Korea, China and Russia to return North Korea to the path of complete and verifiable denuclearisation."
Mr Steinberg is leading a US delegation to Asia this week to consult on how to respond to North Korea's latest nuclear test.