White House says US will not be intimidated by 'bellicose rhetoric' and is fully capable of defending itself and its allies

The White House warned North Korea on Friday that the rapidly escalating military confrontation would lead to further isolation, as the Pentagon declared that the US was fully capable of defending itself and its allies against a missile attack.

After North Korean leader Kim Jong-un declared that rockets were ready to be fired at American bases in the Pacific – a response to the US flying two nuclear-capable B2 stealth bombers over the Korean peninsula this week – the White House blamed Pyongyang for the increased tensions.

"The bellicose rhetoric emanating from North Korea only deepens that nation's isolation. The United States remains committed to safeguarding our allies in the region and our interests that are located there," deputy press spokesman Josh Earnest told reporters travelling with Barack Obama on Air Force One to Miami.

Asked if the joint US-South Korean military exercises and the use of the stealth bombers had fuelled the escalation, Earnest replied: "It's clear that the escalation is taking place from the North Koreans based on their rhetoric and on their actions."

The Pentagon said on Friday that the US would not be intimidated, and was ready to defend both its bases and its allies in the region. Lt Col Catherine Wilkinson, a Pentagon spokesperson, said the US would not be intimidated. "The United States is fully capable of defending itself and our allies against a North Korean attack. We are firmly committed to the defence of South Korea and Japan," she said.

Secretary of state John Kerry will visit the region in a week or so for meetings with Japan, China and South Korea, the State Department said.

North Korea announced that its forces had been placed on high alert on Tuesday but the threats became graver when a picture was published of Kim reiterating the order at an emergency meeting on Friday.

The US Defense Department keeps secret its assessment of the distance North Korea's missiles can reach. But Admiral James Winnefeld, vice chairman of the Joint Chiefs of Staff, said a fortnight ago it had one type of missile capable of reaching the US.

But while defence analysts agreed that while North Korea is theoretically capable of firing a missile, they expressed scepticism about whether its technology is as advanced as it claims and were doubtful about the accuracy in hitting targets.

But there is more concern than usual in Washington compared with previous standoffs with North Korea because Kim is a new leader, young and inexperienced and a largely unknown quantity in the west.

A major worry is that if North Korea was to attack a South Korean ship – it was blamed for the sinking a South Korean vessel in 2010 – or a land target, Seoul has said that it would retaliate this time.

Wilkinson said: "North Korea's bellicose rhetoric and threats follow a pattern designed to raise tensions and intimidate others. DPRK will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in north-east Asia.

"We continue to urge the North Korean leadership to heed President Obama's call to choose the path of peace and come into compliance with its international obligations."

She added: "We remain committed to ensuring peace and stability on the Korean peninsula. This means deterring North Korean aggression, protecting our allies and the complete denuclearisation of the Korean peninsula. The United States will not accept North Korea as a nuclear state; nor will we stand by while it seeks to develop a nuclear-armed missile that can target the United States."

At a Pentagon briefing on Thursday, defence secretary Chuck Hagel said: "There are a lot of unknowns here. But we have to take seriously every provocative, bellicose word and action that this new, young leader has taken so far since he's come to power."

Mark Fitzpatrick, director of the non-proliferation and disarmament programme of the International Institute for Strategic Studies, played down the threat. "North Korea is upping its rhetoric to a world-class level, but it's still just rhetoric. They have no capability to hit the US mainland with anything – except through cyberspace. Their only tested missiles can fly a maximum of 1,600km, less than half the distance to Guam."

Fitzpatrick, who is scheduled to lead a thinktank discussion at the institute's Washington office next Thursday on whether the US policy of patience has run its course and instead it should pursue reunification of the Korean peninsula, said Friday that while North Korea is limited in its ability to hit US targets, it poses a threat to South Korea and Japan.

"Their Scuds and Nodongs can hit anywhere in South Korea and Japan. Using them would be suicidal, of course. The far more likely scenario is a pin-prick attack in the nature of the 2010 attacks. This time, however, South Korea is determined to respond with an eye for an eye, in order to restore deterrence. North Korea's ensuing response could trigger a larger conflagration."

Jim Walsh, a specialist on security and nuclear weapons at MIT, played down the prospect of an attack on the US, but said: "The reason it is scary is: you can get war even when no one intends to have a war. All the sides – South Korea, North Korea and others – are now leaning into each other, and if someone makes a mistake, I am concerned that that mistake will escalate into something larger than anyone expected.

"Suddenly you have a young man in a closed country who has to decide whether he is going to respond to your actions."

The risk was not of a North Korean attack on the US but on South Korea that would bring in the US, he said.

Walsh, who has visited North Korea and has had talks with its officials in Switzerland, Sweden and the US, said the present confrontation felt different because of the harsher rhetoric from North Korea, the secret defence pact agreed by the US and South Korea and the US military drills this week.

"If we are lucky it will all be bluster on everyone's side. That is the good outcome," Walsh said. "The bad outcome is that it is bluster until someone screws up and then war happens."

Michael O'Hanlon, one of the leading military analysts in the US, expressed worries that the US approach of tit-for-tat and imposition of additional permanent sanctions after its third nuclear test could exacerbate the situation. Like Walsh, he sees this confrontation as being different from previous ones.

In an email, O'Hanlon, a security specialist at Washington's Brookings Institution, told the Guardian: "I favour temporary sanctions in response to the third nuclear test, to give Pyongyang an incentive not to provoke again." He argues that by setting a time-limit such as two, three or four years, it could encourage North Korea not to conduct another nuclear test.

"I am talking about automatic sunset provisions with a specific time frame, unless of course there is another nuclear test or another act of violence," O'Hanlon wrote.