"They're doing everything consistent with the launch of a space vehicle on April 4," a US defence official told the Reuters news agency on Thursday, on condition of anonymity.
"We consider the situation as being imminent," a South Korean government official also said.
Pyongyang has said that the rocket is carrying a satellite as part of a peaceful space programme, and has said that the launch will take place between April 4 and April 8.
But the South Korean and Japanese governments allege that North Korea is using the launch to test its Taepodong-2 long-range missile, which is capable of reaching the US.
Pyongyang fired a Taepodong-2 in a July 2006 test session but it exploded shortly after launch. It conducted a nuclear weapons test, its only to date, in October 2006.
The US, South Korea and Japan say another rocket launch by North Korea will violate UN resolutions banning further ballistic missile tests.
But China, which has a veto in the UN Security Council and which has close ties to Pyongyang, is likely to oppose fresh sanctions, as well as the tightening of existing ones against North Korea.
Japan, South Korea and the US have said they do not intend to shoot down the rocket unless it heads towards their respective territories.
North Korea has said that any attempt to destroy the missile will be considered an act of war and that it could restart a plant making arms-grade plutonium if the UN imposes fresh restrictions.