The United States has warned that it will back its allies with the full range of its military capability against any threat from North Korea.
Secretary of State Condoleezza Rice said everyone should know that the US would act fully on its defence treaty obligations to Japan and South Korea.
She was speaking in Tokyo on the first leg of an Asian tour to rally support for enforcing the sanctions.
Pyongyang's test has raised concerns over an arms race in the region.
Ms Rice's hastily arranged trip to Japan, South Korea, China and Russia is aimed at rallying support to implement sanctions fully in line with the UN resolution passed last week.
N KOREA NUCLEAR PROGRAMME
Believed to have 'handful' of nuclear weapons
But not thought to have any small enough to put in a missile
Could try dropping from plane, though world watching closely
South Korea, and to a greater extent China, are still thought to have concerns over some parts of the new UN sanctions.
Ms Rice said she and the Japanese Foreign Minister Taro Aso had pledged "to work together and with other states for the swift implementation and the effective implementation of all of the measures that are contemplated under resolution 1718".
She told a joint news conference that the US had no desire to escalate the crisis but stressed that the US was ready to act on its defence commitments to Japan.
"I want to make sure that everybody understands that the US will fully act on our defence obligations under the mutual defence treaty, " Ms Rice said.
Her comments come amid concern in Washington that the crisis on the Korean peninsula could encourage other Asian nations to develop their own nuclear weapons programmes.
Mr Aso, who had earlier called for a debate on whether Japan should obtain nuclear weapons for itself, was careful to reiterate the government's position that Tokyo has no plans to possess, develop or allow nuclear bombs on its soil
"We do not need to acquire nuclear arms with an assurance by US Secretary of State Rice that the bilateral alliance would work without fault," he said.
Ms Rice and Mr Aso will travel together to South Korea on Thursday.
The US ambassador to South Korea, Alexander Vershbow, says Ms Rice will ask Seoul to expand its role in inspecting ships going to and from the North, to check for weapons of mass destruction.
South Korea has been reluctant to do this for fear it could spark a sea clash with the North.
New test fears
Concerns remain that the North might conduct another nuclear test.
Renewed activity has been reported at last week's test site, and both South Korea and Japan say they have intelligence of possible preparation for a second test.
NEW UN SANCTIONS
Bans sale to, or export from, N Korea of military hardware
Bans sale or export of nuclear and missile related items
Bans sale of luxury goods
Freezes finances and bans travel of anyone involved in nuclear, missile programmes
Allows inspection of cargo to and from N Korea
Stresses new resolution needed for further action
In the past, North Korea is thought to have staged activity at nuclear and missile sites purely to attract attention and step up diplomatic pressure, the BBC's Charles Scanlon in Seoul says.
But analysts in Seoul say Pyongyang will need to conduct further tests in order to develop a credible nuclear deterrent, he adds.
China - the closest country North Korea has to an ally - has appealed to Pyongyang not to escalate tensions still further, urging it to resolve the issue "through dialogue and consultation".
But the North is showing no signs of backing down in the face of international pressure.
On Tuesday, state media dubbed the new UN sanctions against it a "declaration of war"
The authorities also staged a huge sound and light show to mark the 80th anniversary of the party which was precursor to the current rulers, the Communist Workers Party.