US President Barack Obama has told North Korea to end its "belligerent" attitude amid suggestions the secretive regime has developed the capability to mount an nuclear warhead on a ballistic missile for the first time.
On Thursday, Obama told Pyongyang to end its sabre-rattling and warned that he would "take all necessary steps" to protect Americans.
After meeting UN secretary general Ban Ki-moon at the White House, the president said: "We both agreed that now's the time for North Korea to end the kind of belligerent approach that they've been taking and to try to lower temperatures.
"Nobody wants to see a conflict on the Korean peninsula. But it's important for North Korea, like every other country in the world, to observe the basic rules and norms that are set forth, including a wide variety of UN resolutions."
On Friday US secretary of state John Kerry made a unannounced visit to South Korea following talks with G8 leaders in London.
The US is calling on China to help rein in North Korea's actions.
A senior US official told reporters on board Mr Kerry's plane that it was "no secret that China has most leverage, most influence, with North Korea and I think fundamentally we would want them to use some of that leverage because otherwise it is very destabilising and it threatens the whole region."
"China has a huge stake in stability and the continued North Korean pursuit of a nuclear armed missile capability is the enemy of stability. That gives us and the Chinese a very powerful objective in common, namely denuclearisation," the official was quoted by the BBC as saying.
In a statement after the summit foreign secretary William Hague said G8 countries, which include Japan and Russia, condemned North Korea's "current aggressive rhetoric" that would "only serve further to isolate the DPRK".
Kerry's visit comes as speculation mounts that North Korea, led by Kim Jong Un, could launch a mid-range missile designed to reach as far as the US territory of Guam in the Pacific.
On Thursday the US Senate heard an extract from a Defence Intelligence Agency (DIA) report that concluded "with moderate confidence that the North currently has nuclear weapons capable of delivery by ballistic missiles".
However the Pentagon swiftly tried to row back on the revelation, insisting it "would be inaccurate to suggest that the North Korean regime has fully tested, developed or demonstrated the kinds of nuclear capabilities referenced in the passage".
A spokesperson said: "In today's House armed services committee hearing on the department of defence budget, a member of the committee read an unclassified passage in a classified report on North Korea's nuclear capabilities."
Earlier this week former US vice president Dick Cheney told Republicans they should be very wary of recent threats from North Korea. "We're in deep doo doo," Cheney warned the Republican leadership.
Cheney reportedly expressed concern about the lack of intel on young North Korean leader Kim Jung-Un, making him very unpredictable. The former vice president cited his past experience with Saddam Hussein and added, "you never know what they're thinking."