The US is ready to defend itself against anything North Korea might launch, Defence Secretary Chuck Hagel said, as its ally South Korea confirmed its ability to intercept the North's missiles.
At a Pentagon briefing on Wednesday, Hagel told reporters that the US "is fully prepared to deal with any contingency, any action that North Korea may take or any provocation that they may instigate," Hagel added.
He said that the North was "skating close to a very dangerous line" with its bellicose rhetoric and was adding to a "combustible" situation.
"What the US would call crossing a 'dangerous line' would be launching a missile without any kind of pre-warning to shipping and to air traffic that it was about to do so," said Al Jazeera's Harry Fawcett, reporting from Seoul. "In the past, North Korea has, in advance of its rocket launches, given a warning and an idea of the trajectory that its missiles would take.
"This time all indications are that [North Korea is] preparing a missile launch on the east coast [without warning]."
Assuming 'worst case'
General Martin Dempsey, chairman of the Joint Chiefs of Staff, noted that in the absence of statements to the contrary then the confrontational statements have to be taken at face value.
"They have conducted two nuclear tests, they have conducted several successful ballistic missile launches and in the absence of concrete evidence to the contrary, we have to assume the worse case," said Dempsey.
On Thursday, a South Korean defence ministry official said that Seoul has deployed three naval destroyers, an early warning surveillance aircraft and a land-based radar system, as the South braced for what the country's foreign minister said could be a test-fire of a medium-range missile.
Follow coverage of escalating threats in Northeast Asia
At a news conference in Seoul, South Korean defence ministry spokesman Kim Min-seok said he was unable to confirm a Japanese media report that a North Korean missile launcher had been moved into firing position with its rockets facing skyward.
Japanese news agency Kyodo reported earlier on Thursday that the North Korean missile launcher was in a "raised position".
Kim told reporters that South Korea, which raised its military alert status on Wednesday, is fully prepared and has the ability to intercept North Korean missiles.
The reassurances follow weeks of almost daily threats from the North, which are largely seen as rhetoric and an attempt to pressure Washington and Seoul to change their policies towards Pyongyang, as well as to boost the military credentials of their young leader.
Meanwhile, the border between South and North Korea remained tense on Thursday, as more South Korean workers returned from the Kaesong industrial complex, heading south across the Demilitarised Zone that divides the two nations.
Vehicles loaded with goods were seen passing through a checkpoint into the inter-Korea Customs, Immigration and Quarantine area in Paju.
North Korea has pulled out its 53,000 workers from the Kaesong complex, which is its last major economic link with the South.
Looking ahead, North Korea celebrates the birthday of the founding father Kim Il Sung on Monday.
The anniversary is usually a time of military parades and other celebrations.
South Korea's Foreign Minister Yun Byung-se said on Wednesday that "all indications were that North Korea could launch at any time in the run up to the birthday," said Al Jazeera's Fawcett.