SEOUL- North Korea, which this month threatened to carry out a fourth nuclear test, may be closer than previously thought to putting a nuclear warhead on a missile , some experts say, making a mockery of years of U.N. sanctions aimed at curbing such a program.

No one outside the inner circle of North Korea's nuclear program likely knows what advances the country has made. But there has been a shift in thinking by some who study North Korea full time since it conducted a nuclear test in February last year and amid on-off indications it is preparing another.

The isolated and poverty-stricken state, which regularly threatens to destroy the United States and South Korea in a sea of flames, defends its nuclear program as a "treasured sword" to counter what it sees as U.S.-led hostility.

"The field deployment of a nuclear missile is imminent," said Kim Tae-woo, former head of South Korea's state-run Korea Institute

Experts say the delivery vehicle of choice for the North 's first nuclear warhead would most likely be the mid-range Rodong missile , which has a design range of 1,300 km (800 miles).