North Korea recently extended its launch window to the end of the month, citing technical difficulties. Fuel activity last weekend suggested a launch was imminent.
Military sources in South Korea said the North Koreans have removed a rocket from a launch pad to fix technical problems, the Yonhap news agency reports. Min Kyung-joo, a top official at South Korea's Naro Space Center, said withdrawing the rocket from the launch pad suggested North Korea was having major problems with launch preparations.
The North Korean rocket was said to have a range that puts the U.S. West Coast within reach.
An attempt by North Korea to launch a rocket failed in April. Similar launches in 2006 and 2009 coincided with North Korean nuclear tests.
The coming days see the first anniversary of the death of North Korean leader Kim Jong Il and presidential elections in South Korea.