North Korea fired off a long-range rocket on Wednesday, a Seoul military official said, despite international pressure over the plan seen as a test for ballistic missile technology.

The move comes just two days after Pyongyang announced on Monday that it was extending the 13-day launch window by one week until Dec. 29 to fix a "technical deficiency" in the rocket's first-stage control engine module.

In a surprise move, the North fired off the long-range rocket from the Donchang-ri launch site at 9:51 a.m., which was immediately detected by the South Korean military.

"Shortly after the liftoff, Aegis radar system in the Yellow Sea detected the move," a senior military official said.

Since the North's announcement of the rocket launch, the South Korean military has deployed three Aegis warships, equipped with SPY-1 radar with a range of 1,000 kilometers, along the west coast and in southern waters to track the rocket's path.

On Tuesday, satellite imagery showed that the North removed the three-stage rocket from the launch pad, raising speculation that the move was connected with fixing technical problems. (Yonhap News)