The US, Japan and South Korea have deployed missile defence systems and will track a rocket that North Korea says it will launch some time beginning on Monday.
North Korea has notified UN agencies that it would launch a rocket carrying what it called an earth observation satellite between February 8 and February 25.
It has triggered international opposition from some governments that see it as a long-range missile test.
South Korea and Japan have put their militaries on standby to shoot down the rocket, or its parts, if they go off course and threaten to crash onto their territory.
"We will, as we always do, watch carefully if there's a launch, track the launch, [and] have our missile defence assets positioned and ready," US Defence Secretary Ashton Carter said.
"We plan a lot about it. We and our close allies -- the Japanese and the South Koreans -- are ready for it."
South Korea said its Aegis destroyers, its Green Pine anti-ballistic missile radar and early warning and control aircraft Peace Eye are ready.
A US Navy spokesman confirmed the missile tracking ship USNS Howard O Lorenzen arrived in Japan this week but declined to say if it was in response to the North's planned launch.
Coming so soon after North Korea's fourth nuclear test, on January 6, a rocket launch will raise concern that it plans to fit nuclear warheads on its missiles, giving it the capability to strike South Korea, Japan and possibly the US West Coast.
China has told North Korea that it does not want to see anything happen that could further raise tension, Chinese Foreign Minister Wang Yi said, describing it as "a serious situation".
China is the North's sole major ally but it disapproves of its nuclear program. The United States has urged China to use its influence to rein in its neighbour.
A launch would draw fresh US calls for tougher UN sanctions that are already under discussion in response to the nuclear test.
What would likely be an indigenous three-stage rocket will be tracked closely.