North Korea may miss a year-end deadline to declare all its nuclear programs, South Korea's foreign minister said Thursday, after the North warned it would also slow work to disable its atomic facilities due to delayed aid.

North Korea had promised earlier this year to disable its main nuclear complex and give a declaration of all its nuclear programs by the end of the year in return for international aid.

"The timing was initially the end of December but that may go past the target date," Foreign Minister Song Min-soon told reporters.

"When the declaration is made is important, but it should be made in a sincere manner. We are making efforts to achieve a sincere declaration," he said.

The North began disabling key facilities at its nuclear complex north of Pyongyang. However, diplomats have said the North is likely to miss a year-end deadline for the disablement measures, because a key step — removing fuel rods from the reactor — could take several months.

"We are now in a critical juncture," Song said, adding that problems in meeting the deadline lie in both the disablement and disclosure.

On Wednesday, Hyun Hak Pong, a vice director-general at the North's Foreign Ministry, said economic compensation was "being delayed" and that meant the country had "no option but to adjust the speed of the disablement process."

The South Korean foreign minister, however, downplayed the remarks and said the disablement work was going well.

In Washington, U.S. State Department spokesman Gonzalo Gallegos said Wednesday he was not aware of any slowdown in aid and the U.S. expects "further heavy fuel oil shipments and other energy assistance to move forward in the near future."

A Japanese newspaper reported Thursday that the U.S. and North Korea disagree on the amount of plutonium — a key ingredient for atomic bombs — that the communist nation has produced, a figure expected to be included in the declaration.

The regional daily Tokyo Shimbun quoted unnamed U.S. and North Korean officials as saying the North has told the U.S. it has produced about 66 pounds of the nuclear material, considerably less than a U.S. estimates of more than 110 pounds.

Song declined Thursday to address which amount was correct, saying the issue would be dealt with after the North gave its declaration.