The United States extended another offer early this month to hold talks with North Korea, but the communist North has not yet responded to the proposal, diplomatic sources said Sunday.

The offer was made via the North‘s mission to the United Nations before the top nuclear envoys of South Korea, the U.S. and Japan held a trilateral meeting in Seoul to discuss how to deal with Pyongyang, the sources said.

The North’s failure to respond to the dialogue proposal led to the three countries agreeing to ratchet up pressure and sanctions on Pyongyang when their chief nuclear envoys held the three-way talks in Seoul last week, the sources said.

It was not the first time this year the North has rejected a U.S. dialogue proposal.

In January, Ambassador Sung Kim, the chief U.S. nuclear envoy, offered to meet with the North‘s first vice foreign minister, Kim Kye-gwan, in a third country during his trip to the region, but the North virtually rejected the proposal by insisting Kim visit Pyongyang.

The North’s repeated rejection of U.S. dialogue offers could suggest that the regime is trying to wait out the term of the Obama administration while continuing to build up its nuclear and missile capabilities.

The U.S. is trying to create a momentum for dialogue with the North before tensions on the Korean Peninsula rise again in August when South Korea and the U.S. kick off a new round of annual joint military exercises that Pyongyang is sure to protest angrily.

The six-party talks aimed at resolving the North Korean standoff have been stalled since late 2008. North Korea demands the unconditional resumption of negotiations, while the U.S. says that Pyongyang must first take concrete steps demonstrating its denuclearization commitments.

As the six-party talks have been idle, the North has bolstered its nuclear capabilities and stockpile and conducted its second and third nuclear tests, in 2009 and 2013. Some experts now warn that the communist nation‘s nuclear arsenal could increase to 100 bombs by 2020.

Meanwhile, North Korea strongly voiced its opposition to the U.N. Security Council’s handling of recent submarine-launched ballistic missile test, saying that it would prove itself to be a “political tool” of the United States if it were to take issue with Pyongyang while ignoring joint military exercises between South Korea and the U.S.

The North’s ambassador to the U.N., Ja Song-nam, made the claim in a letter sent to the Security Council president last week, calling the drills “real nuclear war games of aggression” aimed at “occupying Pyongyang” to remove the North‘s leadership.

“The recent underwater test-fire by the DPRK of a ballistic missile from a strategic submarine is a legitimate measure of a sovereign state to bolster up its self-defense capability against the provocative military maneuvers of the United States,” the envoy said in the letter, referring to the Democratic People’s Republic of Korea, the official title of North Korea.

“The council will be proved to be a political tool of the high-handed and arbitrary practice of one permanent member” if it only takes issue with the submarine missile launch while ignoring the military exercises, he said.

Ja demanded the council convene an emergency meeting on the joint exercises.

The council can take up the matter only if there is a request from a council member. Last year, North Korea twice asked the Council to deal with the issue, but no formal discussions took place.

Pyongyang has long accused the U.S. and South Korea of plotting to invade the country and using their annual military exercises as part of preparations for it. But Seoul and Washington have rejected the claim, saying the annual maneuvers are purely defensive in nature.

The North’s letter came after South Korea asked the Security Council‘s North Korea Sanctions Committee to look into the North’s submarine missile test to determine whether it violated U.N. resolutions banning Pyongyang from any ballistic missile activity.

The committee reported to the council last week that it will launch an investigation.

On May 9, North Korea claimed it successfully carried out an SLBM test underwater, renewing tensions on the Korean Peninsula amid concern that the North‘s SLBM capability, if fully developed, would pose a serious threat because its mobile nature would make it very difficult to detect preparations for a launch.

From news reports