South Korea, the United States and Japan expressed disappointment Monday as an end-of-year deadline for North Korea to declare all its nuclear programs under an aid-for-disarmament deal appeared set to pass unmet.

"Our government urges North Korea to faithfully declare all nuclear programs at an early date and complete disablement steps without delay," South Korea's Foreign Ministry said in a statement, adding it was "regrettable" that the country has so far failed to provide the promised list.

The three countries, along with China and Russia, have been pushing North Korea to abandon its nuclear programs in a series of negotiations that began in 2003 — a process that achieved its most impressive results to date in 2007.

In February, Pyongyang promised to abandon its nuclear ambitions in return for energy aid and political concessions. In October, it vowed to disable its nuclear facilities and declare its programs by the end of the year. Watched by U.S. experts, the North began disabling the reactor last month.

Diplomats, however, have said for weeks that the country was likely to miss the year-end deadline for disablement because a key technical step — removing fuel rods from the reactor — could take several months.

They had held out hope, however, that the disclosure commitment might still be met, but even those expectations were lowered, with South Korean Foreign Minister Song Min-soon saying last week that the announcement "may go past the target date."

Last week, a North Korean official indicated that the country would slow its disablement work because it was dissatisfied over the delivery of aid.

The North promised to carry out the agreement in return for the equivalent of 1 million tons (910,000 metric tons) of oil from South Korea, the U.S., China and Russia. South Korean media have reported that Pyongyang has so far received 150,000 tons of that total.

The U.S., which said last week it was unaware of any slowdown in aid shipments, expressed disappointment at the delay in the promised declaration.

"There has been no last minute change," Tom Casey, deputy spokesman for the State Department, said Monday. "It is unfortunate but we're going to keep on working on this. We're still committed to getting a declaration and we want that declaration to be full and complete."

In Tokyo, the Foreign Ministry on Monday also called the lack of a declaration "unfortunate" and urged it be released "at the earliest possible date." The Chinese and Russian governments, also members of the six-party forum, were silent.

No sanction or penalty has been stipulated by any members of the six-party forum if the North fails to make the declaration on time.

North Korean state media made no mention Monday of the declaration.

The country's communist party newspaper, however, took a swipe at the U.S., accusing it of flexing its military muscle in South Korea.

"Dialogue and clamor for war cannot stand together," the Rodong Sinmun said in a commentary, according to South Korea's Yonhap news agency. "The U.S. should abandon its hostile policy toward the North."

The reasons for the delay in declaring the programs appear related to the country's suspected uranium enrichment program and differences with Washington over how much plutonium it has produced.

Song, South Korea's top diplomat, said last week that more consultation was required on the uranium program, while a Japanese newspaper reported that Pyongyang and Washington disagree on the amount of plutonium the North has produced.

South Korean analysts, however, pointed to another issue, saying that North Korea appeared to be unhappy with slowness by the U.S. in removing the country from a list of terror-sponsoring states.

Yang Moo-jin, a professor at Seoul's University of North Korean Studies, said North Korea's recent behavior suggests there may be a lull in the denuclearization process for a few months.

"Instead of stressing the North's declaration, the U.S. should show some sincerity over the terrorism list," Yang said.

In early December, South Korea's Song said the U.S. was making preparations to remove Pyongyang from the list if Pyongyang's nuclear declaration was acceptable.

Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University, said the delay shows again that North Korea will move forward only when it is given promised aid and other concessions.

He said, however, that the denuclearization process "won't be undermined" and the delay "cannot be seen as a collapse of the agreement."