Foreign ministers from the G8 group of nations have condemned in the "strongest possible terms" North Korea's nuclear weapons and ballistic missile programmes.
Tensions have risen on the Korean peninsula in recent weeks.
Meanwhile, a Pentagon spy report concluded "with moderate confidence" that North Korea had the capability to launch nuclear-armed missiles.
But their reliability would be low, the Defence Intelligence Agency (DIA) said.
It is thought to be the first time that the agency has acknowledged North Korea's capability to produce warheads small enough to fit onto a missile.
The report's conclusion was made public by Republican Doug Lamborn as he questioned senior Pentagon officials about North Korea's nuclear weapons programme during a hearing of the House of Representatives Armed Services Committee.
"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however the reliability will be low," Mr Lamborn said, quoting directly from the report released in March.
The study's conclusion was erroneously marked unclassified, an unnamed US official later told the Associated Press news agency.
In a news conference after the G8 summit, UK Foreign Secretary William Hague said that "if the DPRK [North Korea] conducts another missile launch or nuclear test, we have committed ourselves to take further significant measures".
The Group of Eight nations comprises the US, Britain, France, Germany, Italy, Japan, Canada and Russia.
Britain currently holds the rotating chairmanship of the G8 and the talks are a prelude to the annual G8 summit later this year in Northern Ireland.
Correspondents say Japan, present at the talks, had been looking for a strong statement of solidarity over Korea.
North Korea has been making bellicose threats against South Korea, Japan and US bases in the region.
Mr Hague said the ministers condemned North Korea's "current aggressive rhetoric", saying it would "only serve further to isolate the DPRK".
Later on Thursday, US President Barack Obama also called on North Korea to end its "belligerent approach".
He added that the US would take "all necessary steps" to protect its people, while stressing that "nobody wants to see a conflict on the Korean peninsula".
BBC diplomatic correspondent James Robbins says ministers agree that the combination of warlike threats from North Korea and preparations for new missile tests amount to dangerous provocation.
South Korea has raised its alert level amid indications that the North is preparing for a missile test.
Pyongyang has moved two Musudan ballistic missiles to its east coast. Estimates of their range vary, but some suggest it could travel 4,000km (2,500 miles).
A missile therefore has the potential of hitting US bases on Guam, although it is not known whether the Musudan has been tested before.
North Korea has increased its fiery rhetoric following fresh UN sanctions imposed after its third nuclear test and joint military manoeuvres by the US and South Korea.
The North says it will restart a mothballed nuclear reactor, has shut an emergency military hotline to the South and has urged countries to withdraw diplomatic staff, saying it cannot now guarantee their safety.
However, in the past few days North Korea's media appear to be in more of a holiday mood, due to the approach of Kim Il-sung's birthday on Monday - a potential launch date for a new missile test.
The G8 ministers also pledged to work to end sexual violence in conflict, calling for urgent action to address "comprehensively" the "culture of impunity" in conflict zones.
Mr Hague said he was "delighted" that ministers had agreed on plans to tackle "the horrific use of rape and sexual violence as a weapon of war in conflicts around the globe", which he described as "one of the greatest and most persistent injustices in the world".
Mr Hague said the G8 had "committed to the development of a comprehensive international protocol on the investigation and documentation of rape and sexual violence in conflict".
The UK announced £10m ($15.4m) of fresh funding to supports efforts against sexual violence.
In a statement welcoming the moves, the Save the Children charity said: "The majority of victims of sexual violence, especially in conflict situations, are children so we must ensure these funds reach the most vulnerable children as a matter of urgency."
The UN special envoy for refugees, Angelina Jolie, said that wartime rape should not be regarded as inevitable, saying: "It can be prevented and must be confronted.
"Finally we have some hope to offer victims."
On Syria, Mr Hague admitted that "the world has failed so far in its responsibilities, and continues to do so", adding that divisions over the conflict continue.
"This is on track to be the biggest humanitarian catastrophe of the 21st Century so far," he added.
Ministers called for greater humanitarian assistance to Syrians affected by the conflict.
They affirmed their support for a "political transition", but did not mention any punitive measures against President Bashar al-Assad.
Fresh evidence of links between some opposition fighters and al-Qaeda has made it even harder for governments to decide a course of action, correspondents say.
G8 ministers met Syrian opposition figures on Wednesday on the sidelines of the two-day forum.
More than 60,000 people are estimated to have died since the uprising against the government of President Assad began in March 2011.
The London talks were also the first chance for G8 ministers to discuss face-to-face the failure of last week's meeting in Almaty, Kazakhstan, on curbing Iran's nuclear programme.
Mr Hague called that failure "disappointing".
"We will continue to work with the twin-track approach of sanctions and negotiations, but... the window of diplomacy will not remain open forever," he went on.
Tehran says it only wants to produce energy but the US and its allies suspect it is trying to develop a nuclear weapon.
Burma, Somalia and cyber-security were also topics on the agenda.