In this story:

• Underground complex provokes suspicion
• Allies consider N. Korea's actions
• Related stories and sites

PYONGYANG, North Korea (CNN) -- Amid rising tensions, North Korea is reportedly growing increasingly frustrated that the United States, Japan and South Korea are not keeping their end of a landmark 1994 deal.

Under the agreement, North Korea promised to freeze its nuclear program in return for U.S. pledges to provide it with fuel oil, help in building two light-water reactors and, eventually, an end to economic sanctions.

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The United States and North Korea's neighbors believed the nuclear program in the North was designed to produced large amounts of weapons-grade plutonium.

In the agreement, the United States said it would help supply two light-water reactors for electric power generation if North Korea would halt its weapons program.

Four years later, however, shipments of fuel oil to North Korean ports are way behind schedule, and construction on the new reactors has ground to a halt one year after a ceremonial groundbreaking.

In the United States, Congress has been reluctant to fund the agreement with North Korea, and there has been little easing of U.S. sanctions.

Economically strapped Japan and South Korea, which originally agreed to cover most of the cost of the reactors, are finding it hard to hand over billions of dollars now.

Diplomats in North Korea have told CNN's Mike Chinoy that Pyongyang's frustration over the unfulfilled pledges is a key reason for provocative actions in recent weeks.

These include the launch of a satellite-carrying missile capable of reaching Japan and Alaska and reports that a new underground complex near North Korea's existing nuclear facility may be under construction.

"Apparently, the North Koreans have been building two underground sites which could potentially be used for nuclear weapons development," Lee Chong-min of Yonsei University in South Korea said.

The North Koreans insist the new sites are for civilian purposes.

The rocket launched by North Korea on August 31

U.S., South Korean and Japanese officials met Monday at the State Department in Washington to discuss North Korea's missile launch, among other topics.

Although South Korea and Japan had agreed in principle to pay most of the $4.5 billion for the two reactors promised North Korea, Tokyo balked at signing a contract after the North's missile launch on August 31.

Despite delays by Congress to fund the deal, Washington believes the agreement must be kept, State Department spokesman James Rubin said Monday.

"Let's remember what we're dealing with here," he said. "The very real and tangible possibility that in the absence of a framework, we would have a crisis with North Korea and the prospect of a nuclear-armed North Korea in Asia, which should frighten both those countries there and any sane citizen."

The United States said Thursday it had averted a collapse in the fragile dialogue with North Korea, reviving cooperation on freezing its nuclear program and on reining in its missiles threat.

CNN's Hong Kong Bureau Chief Mike Chinoy and Reuters contributed to this report.