A rocket is fired during a drill of drone planes assaulting targets and a firing drill of self-propelled flak rocket destroying ''enemy'' cruise missiles coming in attack in low altitude, conducted by the air force and air defence artillery units of the Korean People's Army in an undisclosed location in this picture released by the North's official KCNA news agency in Pyongyang March 20, 2013. KCNA said this picture was taken on March 20, 2013.
Three drones that crashed in South Korea had onboard flight programming that showed they were launched from North Korea and were meant to return after flying over military installations in the South, the defense ministry in Seoul said on Thursday.
South Korean and U.S. officials jointly examined the three drones that were recovered in three different locations near the Korean border over a two-week period starting in late March.
The second was discovered soon after a three-hour artillery barrage between North and South Korea in waters near a disputed maritime border.
The drones' penetration of South Korean airspace raised questions about its air defense capabilities while Pyongyang clings to its hard-line stance against Seoul.
"North Korea's action is a clear military provocation that violates the armistice and the South-North non-aggression agreement," the South's defense ministry said in a statement.
Pyongyang has denied any involvement, calling the South's charge a fabrication.
In April, North Korea proposed a joint probe with the South but Seoul rejected the proposal.
South Korea's defense ministry also said in April some of the parts in the recovered drones were manufactured in China, Japan, the Czech Republic and the United States, but it offered no further details.
Photographs unearthed by the North Korea Tech blog showed a drone made by a Chinese company with an almost identical size and shape to some of the drones found in South Korea.
South Korea's defense ministry said it was aware of the Chinese-made drone and had sought explanations from the Chinese government.
Repeated calls by Reuters to Taiyuan Hangyou Hangkong Technology Co. Ltd, the company that produces the drones, were not answered.
Chin's foreign ministry also did not respond immediately to requests for comment.
So far this year North Korea has test-fired medium-range ballistic missiles, threatened to conduct a nuclear test, and fired more than 500 artillery shells that landed in disputed waters between the two Koreas.
Pyongyang has also recently conducted engine tests for an intercontinental ballistic missile that could potentially deliver a nuclear warhead to the United States, a U.S. think tank said on Friday.
North Korea released TV footage last year of practice drones which had been modified to crash into predetermined targets, but it is not believed to operate drones capable of air strikes or long-range surveillance sorties.
North Korea's state media said last year leader Kim Jong Un had supervised a drill of "super-precision" drone attacks on a simulated South Korean target.
Although the North has one of the world's largest standing armies, much of its equipment consists of antiquated Soviet-era designs. It has focused its resources on developing nuclear and long-range missile programs.