The spy agency, which denies meddling in domestic politics, declined to comment on the opposition’s claim.

North Korea’s planned satellite launching, scheduled for sometime between Thursday and next Monday, had already deepened international tensions. The United States and its allies have warned that launching a missile would further isolate the North by prompting more severe enforcement of international sanctions.

North Korea has repeatedly signaled its determination to launch the satellite.

The South Korean government spokesman also confirmed that the latest satellite images showed all three stages of a rocket placed on the North’s launching pad. In a dispatch from the Tongchang-ri launching site, The Associated Press also confirmed that the rocket was in position. The A.P.’s journalists were among the foreign reporters invited to observe the launching.

Last month, North Korea’s Foreign Ministry warned Washington that the cancellation of promised food aid in retaliation for the launching would automatically kill a Feb. 29 deal in which North Korea agreed to a moratorium on uranium enrichment and long-range missile and nuclear tests.

Image A dirt pile at a tunnel in Kilju was reported as evidence. Credit... The New York Times

North Korea’s two previous nuclear tests involved plutonium devices. If the country attempts a third test, analysts have said, it may use a nuclear device fueled by highly enriched uranium. North Korea revealed an advanced, industrial-scale uranium enrichment plant in 2010, though it is unclear how much highly enriched uranium it has acquired.

North Korea says it has been planning its satellite launching for several years to coincide with national celebrations of the 100th birthday this year of Kim Il-sung, its revered national founder and the grandfather of its new leader, Kim Jong-un. But American officials say they believe that North Korea’s satellite program is a cover for developing intercontinental ballistic missiles that might someday be able to deliver nuclear warheads.