North Korea has announced it will restart its nuclear reactor at Yongbyon, which was shut down in 2007.
The state-run KCNA news agency reported on April 2 that Pyongyang will "readjust and restart" all the complex's facilities, which include a uranium-enrichment plant.
UN Secretary-General Ban Ki-moon said at a news conference during an official visit to Andorra that North Korea appeared to be "on a collision course with the international community."
At a news briefing in Seoul, South Korean Foreign Ministry spokesman Cho Tai-young said Pyongyang's latest announcement was an unfortunate development.
"If the report is true, it is really regrettable," he said. "North Korea should keep their promise and agreements and they should keep to denuclearization of the Korean Peninsula. Our government will closely monitor the situation."
The announcement about Yongbyon is the latest in a series of tension-raising measures by Pyongyang after its third nuclear test in February. That test prompted a new round of UN sanctions against the country.
The Yongbyon reactor is believed to have produced the plutonium for North Korea's first two nuclear-weapons tests in 2006 and 2009.
The reactor was partially dismantled after a 2007 agreement with the international community under which Pyongyang was given 50,000 tons of fuel oil.
In a show of strength, Washington has moved advanced aircraft and warships around the Korean Peninsula as a part of ongoing military exercises with South Korea that began last month.
The exercises have provoked a series of threats from the communist state. In recent weeks, Pyongyang announced its long-range missile units were targeting U.S. bases in the Pacific and in the mainland United States. It said artillery units were also aiming at U.S. and South Korean bases.
On March 30, North Korea said it had entered a "state of war" with South Korea. A statement from Pyongyang said any military provocation would result "in a full-scale conflict and a nuclear war."
Despite the harsh rhetoric, White House spokesman Jay Carney said that Washington had not seen any signs of increased military activity in North Korea.
"We are not seeing changes to the North Korean military posture, such as large-scale mobilizations and positioning of forces," Carney said.
But he said Washington was "monitoring the Korean situation very diligently."
Earlier on April 2, North Korean media published a speech by leader Kim Jong Un in which he said the country's nuclear program supported its economic development.
With reporting by Reuters, AP, and AFP