US Secretary of State Condoleezza Rice has welcomed a deal reached with North Korea over its nuclear programme during six-nation talks in Beijing.
Pyongyang has agreed to take the first steps towards nuclear disarmament, promising to shut down its main nuclear reactor in return for fuel aid.
The US and Japan have also pledged to begin talks with North Korea on building closer ties.
Ms Rice said the agreement was "not the end of the story" but was a good start.
"The goal is the complete, verifiable and irreversible de-nuclearisation of the Korean peninsula," she said. "This is a good beginning to that effort."
She rejected a suggestion that the deal would be seen as a reward for bad behaviour by nations such as Iran, which is also under pressure over its nuclear programme.
"Why shouldn't it be seen as a message to Iran that the international community is able to bring together its resources," she said.
'Only one phase'
Reading out the agreement, China's chief envoy Wu Dawei said the deal was "favourable for the peace process in north-east Asia and for the improvement of ties between relevant countries".
Delegates from the two Koreas, the US, China, Japan and Russia had spent from Thursday until late Monday night in Beijing, hammering out the final details.
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
Under the agreement, Pyongyang has pledged to close its Yongbyon reactor within 60 days, in return for 50,000 metric tons of fuel aid or economic aid of equal value.
The closure of Yongbyon will be verified by international inspectors.
The North will eventually receive another one million tonnes of fuel oil or an equivalent when it permanently disables its nuclear operations.
Pyongyang has not commented officially on the deal. However, North Korea's official news agency, KCNA, described it as being based on the "temporary suspension" of its nuclear activities.
The US has agreed to begin the process of removing North Korea from its list of terror states and establish diplomatic relations.
Japan will also discuss normalising relations with the North.
Chief US negotiator, Christopher Hill, said the agreement reached this week was "only one phase of denuclearisation. We're not done".
One of the topics that looks set to be left for later discussion is the fate of any nuclear weapons the North already possesses.
Signs of progress
US President George W Bush said he was "pleased" with the agreement.
White House spokesman Tony Snow called it "a very important first step" towards denuclearising the Korean peninsula.
However, he warned that North Korea still faced the possibility of sanctions if it did not abide by the terms of the deal.
Other US figures had earlier voiced scepticism.
John Bolton, former US ambassador to the United Nations, said North Korea should not be rewarded with "massive shipments of heavy fuel oil" for only partially dismantling its nuclear arsenal.
And while Japan has approved the joint agreement, Foreign Minister Taro Aso was quoted as saying that Tokyo would not provide aid as there had been no progress on the issue of Japanese nationals abducted by the North in the 1970s and 80s.
South Korean President Roh Moo-hyun welcomed the "good results", but added: "All necessary measures must be taken immediately in order to translate the agreement into reality."
But despite the difficulties ahead, analysts say this deal is an important sign of progress, after more than three years of talks.
The previous deal, agreed in September 2005, rapidly fell apart over differences between North Korea and the US over implementation.
The North Korean nuclear issue has become even more pressing in recent months, after Pyongyang conducted its first atomic test in October.