North Korea's missile capability is "fairly rudimentary", US Vice-President Dick Cheney has said.
The new Taepodong missile could reach Alaska (archive picture)
Mr Cheney was responding to calls from former top US defence officials urging that North Korea's missile be destroyed before it can be test-fired.
Several US officials have stepped back from threatening pre-emptive action against the North, while warning it can expect "consequences" if it goes ahead.
North Korea is thought to have prepared a Taepodong-2 missile for launch.
The untested missile has an estimated range of up to 6,000km (3,730 miles) - potentially reaching Alaska.
However, a South Korean official has insisted the launch is not imminent, and no substantial moves towards the launch of the missile from Musandri - on the North's north-eastern coast - have been detected in recent days.
In an opinion piece in the Washington Post on Thursday, William Perry and Ashton Carter urge the US administration to act before a "mortal threat" can develop.
It should "immediately make clear its intention to strike and destroy the North Korean Taepodong missile before it can be launched", say the two men - respectively secretary of defence and assistant defence secretary under former President Bill Clinton.
Mr Cheney rebuffed that suggestion in an interview with CNN.
It was "fair to say that the North Korean missile capabilities are fairly rudimentary," he said.
"I mean, they've been building Scuds and so forth over the years, but their test flights in the past haven't been notably successful. But we are watching it with interest and following it very closely."
He said the US was addressing the issue "in a proper fashion", while warning the North that "if you're going to launch strikes at another nation, you'd better be prepared to not just fire one shot".
The US and its Asian allies have warned North Korea repeatedly against launching a long-range missile, but on Thursday several US officials sought to bring down the temperature of the debate surrounding the test-firing.
"A pre-emptive strike is a little more dramatic than I expect would happen," Peter Rodman, assistant defence secretary for international security affairs, told a congressional committee.
A Pentagon spokesman, Bryan Whitman, told reporters President Bush was using diplomatic rather than military channels to resolve the situation.
However, officials are still warning of "consequences" should North Korea go ahead with the test.
Mr Rodman said a launch would be a "provocation... which would have to have some consequences".
And an unnamed senior defence official told news agencies said that the US would use "any capabilities it had if it could protect the American people".
He said the US would not necessarily use its missile defence system to destroy a missile heading for the open sea - a statement which some interpreted as the clearest indication yet that the US has activated its missile defence system.
The Pentagon has refused to confirm or deny speculation that the system - which is still in development - would be used to bring down a North Korea missile.
South Korean calm
Meanwhile, South Korean Defence Minister Yoon Kwang-ung told a parliamentary committee that "there are many processes to go through before firing such a missile".
"Given this," he said, an imminent launch of the missile "is not the case".
According to South Korea's Yonhap news agency, Pyongyang's envoy to the UN, Han Song-ryol, has indicated his country is willing to delay the test for talks with the US.
"The United States says it is concerned about our missile test launch," Mr Han is quoted as saying. "Our position is, 'Okay then, let's talk about it'."
However, the US - which has consistently rejected Pyongyang's calls for bilateral talks - has said it is not ready to be forced to the negotiating table with the threat of a missile test.
The BBC's Charles Scanlon in Seoul says Pyongyang's immediate neighbours are concerned Washington may over-react to the missile threat.
South Korea and China both tend to see the threat from North Korea as political rather than military, he says.