SEOUL/TOKYO (Reuters) - North Korea on Saturday demanded the conclusion of a peace treaty with the United States and a halt to U.S. military exercises with South Korea to end its nuclear tests.

Japan's Vice Foreign Minister Akitaka Saiki (C), U.S. Deputy Secretary of State Antony Blinken (L), and South Korean First Vice Foreign Minister Lim Sung-nam attend a joint news conference during their trilateral meeting at the foreign ministry's Iikura guest house in Tokyo, Japan, January 16, 2016. REUTERS/Kimimasa Mayama/Pool

But U.S. Deputy Secretary of State Antony Blinken said Pyongyang needed to demonstrate by its action that it was serious about denuclearization for any dialogues to start.

“We now have unfortunately a decade during which North Korea has totally reversed its obligations to international community, when it comes to missile and nuclear programs,” Blinken told a news conference in Tokyo.

“So it’s very hard to take any of their overtures very seriously, particularly in the wake of their fourth nuclear test,” he said, after meeting his counterpart from Japan and South Korea.

North Korea said on Jan. 6 it had tested a hydrogen bomb, provoking condemnation from its neighbors and the United States.

The isolated state has long sought a peace treaty with the United States, as well as an end to the exercises by South Korea and the United States, which has about 28,500 troops based in South Korea.

“Still valid are all proposals for preserving peace and stability on the peninsula and in Northeast Asia including the ones for ceasing our nuclear test and the conclusion of a peace treaty in return for U.S. halt to joint military exercises,” North Korea’s official KCNA news agency cited a spokesman for the country’s foreign ministry as saying early on Saturday.

Slideshow ( 3 images )

But asked if the United States would consider a halt to joint exercises, U.S. State Department spokesman John Kirby said it had alliance commitments to South Korea.

“We are going to continue to make sure the alliance is ready in all respects to act in defense of the South Korean people and the security of the peninsula,” he told a regular news briefing.

Vice foreign ministers from the United States, Japan and South Korea also agreed to seek tough U.S. sanctions on Pyongyang, calling for China to take more actions.

China is North Korea’s main economic and diplomatic backer, although relations between the Cold War allies have cooled in recent years.

The two Koreas remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty.

Experts have expressed doubt that the North’s fourth nuclear test was of a hydrogen bomb, as the blast was roughly the same size as that from its previous test, of a less powerful atomic bomb, in 2013.

Pyongyang is under U.N. sanctions for its nuclear and missile programs.

Blinken said that Pyongyang should look to the example of Iran.

Iran’s foreign minister said international sanctions on the country will be lifted on Saturday when the United Nations nuclear agency declares Tehran has complied with an agreement to scale back its nuclear program.

“What made that agreement (with Iran) possible was the decision by Iran to freeze, and in some respects roll back, its nuclear program, in order to allow time and space to see if we could negotiate a comprehensive agreement.”