A US counter-proliferation official, who spoke on condition of anonymity, told the AFP news agency that reports that a rocket had been placed on a launch pad are "accurate".
The official said the missile was believed to be a Taepodong-2, a long-range missile that experts believe could reach as far as Alaska.
On Wednesday the US secretary of state warned that Washington would take the issue to the United Nations if the launch goes ahead.
Speaking on a visit to Mexico, Hillary Clinton said a launch for any purpose would be a violation of a UN Security Council resolution.
"This provocative action in violation of the United Nations mandate will not go unnoticed and there will be consequences," Clinton told reporters.
|Neighbouring countries are on alert ahead of the planned launch [GALLO/GETTY]
In 2006, the Security Council passed a resolution banning North Korea from conducting long-range missile tests, or using a rocket to send a satellite into space.
However, Brian Myers, a North Korea analyst at South Korea's Dongseo University, told Al Jazeera that the US can do very little if North Korea decides to conduct a missile test.
"Despite the rhetoric heard about 'consequences', there is very little that America can do," he said.
"The North Korean government is also vying for attention here. The economy is in ruins, and they want to give their people something to rally around."
On Tuesday, North Korea warned that six-party talks on its nuclear programme would collapse if new sanctions are imposed to punish it for the launch.
The talks include representatives from the United States, Japan, Russia, North Korea, South Korea and China.
Taro Aso, the Japanese prime minister, said on Wednesday the country's security council, will meet this week to prepare options for possibly shooting down the North Korean rocket if it enters its territory.
North Korea said it would regard any attempt to shoot down its rocket as an act of war.
The last time North Korea launched a Taepodong-2 was in 2006. The missile failed and exploded less than a minute after launch.