The US has defended its robust response to military threats from North Korea amid criticism that both sides are to blame for the escalating rhetorical standoff.

Pyongyang ratcheted tensions in the region to new levels on Thursday by reportedly moving a medium range missile to its eastern coast after warning it has been authorised to attack the US using "smaller, lighter and diversified" nuclear weapons. This followed steps by Washington to move interceptor missiles and warships to the region to defend against possible attack by the North.

But the US State Department responded to questions from reporters about the danger of a downward spiral by insisting that it had no choice but to respond in this way.

"When you have a country that is making the kind of bellicose statements and taking the kind of steps that they have, you have to take it seriously and you have to take steps to defend the US and its allies," said spokeswoman Victoria Nuland.

"The ratcheting up of tension on the DPRK side was the cause of us shoring up our defensive posture."

However, Washington also announced fresh moves to seek a diplomatic solution to the crisis, revealing that it had made a phone call to officials in Beijing to ask them to press North Korea to tone down its rhetoric. Secretary of state John Kerry is due to meet his counterpart in Beijing on a scheduled visit to Asia, and South Korean leader Park Geun-hye is also due to meet President Obama in Washington for talks next week.

The State Department said it was optimistic that the international alliance calling on North Korea to abandon its nuclear weapons programme would hold firm and "recognise the threat we share is common and that we are stronger if we work together".

Nuland also said it was within Pyongyang's ability to return to the international community and end sanctions. "This does not have to get hotter," she said. "They just have to comply with their international obligations."

The past week has seen a growing series of threats in the most significant bout of sabre-rattling since an artillery exchange between North and South in 2010. North Korea also closed the shared Kaesong industrial zone and vowed to restart a mothballed nuclear plant.

Although officials in South Korean stress they do not think an attack is imminent, the risk of accidental conflict is high after North Korea withdrew from a system of hotlines. South Korea also adopted more proactive deterrence strategy after attacks by the North in 2010, threatening to respond with disproportionate force to any future provocation.

South Korea's defence minister claimed early on Thursday that Pyongyang had moved a missile with "considerable range" to its east coast, but said there were no signs that North Korea was preparing for a full-scale conflict despite the continuing standoff.

The confirmation from Kim Kwan-jin came hours after North Korea's military announced it had been authorised to attack the US using "smaller, lighter and diversified" nuclear weapons.

Kim said he did not know why the North had moved the missile, but suggested it "could be for testing or drills".

He dismissed Japanese media speculation that the missile could be a KN-08, which is believed to be a long-range missile that – if operable – could hit the US.

Kim told a parliamentary committee meeting that although the missile had considerable range, it was not sufficient to hit the US mainland.

His description could suggest a missile known as the Musudan, which has a range of 3,000km (1,800 miles). That would make Japan and South Korea potential targets along with US bases in both countries.

There are doubts, however, about the missile's accuracy and range, and some suspect that long-range missiles unveiled by Pyongyang at a parade last year were actually mock-ups.

Kim said that if North Korea were preparing for a full-scale conflict, there would be more signs of the mobilisation of troops and supplies.

So far, he said, South Korean military officials had found no evidence of such preparations.

"[North Korea's recent threats] are rhetorical threats," he said. "I believe the odds of a full-scale provocation are small."

He did, however, add that North Korea might mount a small-scale provocation as it did in 2010, when it shelled a South Korean island, killing four people.

In a further reminder of historic tensions, the White House announced on Thursday that President Obama would next week be making a posthumous medal of honour award to a Korean war veteran captured in 1950.

Computer hackers apparently also broke into at least two of North Korea's government-run social media accounts on Thursday.

The North's Uriminzokkiri Twitter and Flickr accounts stopped sending out content typical of that posted by the regime in Pyongyang, such as photos of North's leader Kim Jong-Un meeting with military officials.

Instead, a picture posted Thursday on the North's Flickr site showed Kim's face with a pig-like snout and a drawing of Mickey Mouse on his chest.

Underneath, the text read: "Threatening world peace with ICBMs and Nuclear weapons/Wasting money while his people starve to death."

Another posting said "We are Anonymous" in white letters against a black background. Anonymous is a name of a hacker activist group. A statement purporting to come from the attackers and widely circulated online said that they had compromised 15,000 user records hosted on Uriminzokkiri.com and other websites. The authenticity of the statement couldn't be confirmed, but the North's official website did not open Thursday.