North Korea is building a large nuclear reactor and a reprocessing plant that could be used to make nuclear weapons within four years, a South Korean newspaper reported Wednesday.

The Segye Times, quoting an unidentified source described as being well-informed on the north's nuclear development, said the new reactor is being built near Yongbyon in northern North Korea and is to be completed in 1992.With the construction of the reactor, the country's third, North Korea would have the capacity to produce 62 pounds of plutonium a year by 1994 or 1995, said the paper, a conservative national daily.

It said this would be enough plutonium to produce two or three bombs of the size the United States dropped on Nagasaki, Japan, at the end of World War II.

A uranium refinery to produce enriched uranium for nuclear fuel is being constructed near Pyongsan, located about 25 miles from the heavily armed Korean border, the paper said.

It was impossible to get immediate comment on the report from Pyongyang because North Korea is closed to outsiders.

There have been heightened international concerns that increasingly isolated North Korea will be able to produce nuclear weapons by the mid-1990s.

The Soviet Union, the United States, Japan, China and South Korea have called on North Korea to sign a nuclear safeguards treaty and open its nuclear facilities to inspection.

North Korea has refused, citing nuclear weapons allegedly kept by the United States in pro-Western South Korea, its archenemy.

In Geneva on Wednesday, Ri Tchol, North Korea's U.N. ambassador, said his country has "no intention, no capacity, no necessity to produce nuclear arms."

But he again demanded that the United States remove any nuclear weapons from South Korea before his government signs the Nuclear Non-Proliferation Treaty that mandates inspections.

Lee Joung-binn, South Korea's assistant minister for foreign affairs, told a foreign news briefing Tuesday that unspecified "new construction of substantial magnitude is now under way" in North Korea.

Lee said the north's refusal to put its nuclear facilities under international safeguards "cannot but be interpreted as indicating its intention for nuclear weapons development."

South Korea's defense minister recently said Seoul might launch a commando raid against North Korean nuclear facilities if the Pyongyang government did not allow international inspections.