By Jack Kim

SEOUL (Reuters) -North Korea denounced on Monday what it called a move by the United States to introduce a nuclear missile submarine to waters near the Korean peninsula, saying it creates a situation that brings a nuclear conflict closer to reality.

North Korea also claimed U.S. reconnaissance planes recently violated its air space near the east coast, quoting an unnamed spokesperson of its Ministry of National Defence in a statement carried by the official KCNA news agency.

"There is no guarantee that a shocking incident where a U.S. air force strategic reconnaissance plane is shot down over the East Sea will not happen," the spokesperson said.

The statement cited past incidents of the North shooting down or intercepting U.S. aircraft at the border with South Korea and off the coast. North Korea has often complained about U.S. surveillance flights near the peninsula.

The moves by the United States to introduce strategic nuclear assets to the Korean peninsula is a blatant nuclear blackmail against North Korea and regional countries and presents a grave threat to peace, KCNA said.

"It is up to future U.S. actions whether an extreme situation arises in the Korean peninsula region that nobody wants, and the United States will be held fully responsible if any unexpected situation occurs," it said.

A U.S. nuclear-powered cruise missile submarine arrived at the port of Busan in South Korea last month.

In April, the leaders of South Korea and the United States agreed a U.S. Navy nuclear-armed ballistic missile submarine will visit South Korea for the first time since the 1980s but no timetable has been given for such a visit.

It was part of a plan to boost the deployment of American strategic assets aimed at a more effective response to North Korea's threats and weapons tests in defence of its ally South Korea, as agreed by the two leaders.

South Korean President Yoon Suk Yeol said it was time to show "the international community’s determination to deter North Korea’s nuclear weapons program is stronger than North Korea’s desire to develop nuclear weapons,” in written comments to the Associated Press published on Monday.

Yoon is scheduled to attend the NATO summit in Lithuania this week where he is expected to seek greater cooperation with NATO members over North Korea's nuclear and missile threats, his office has said.

The move to sail nuclear submarines has created a "very dangerous situation that makes it impossible for us not to realistically accept the worst-case scenario of a nuclear confrontation," the North Korean statement said.

In June, a U.S. B-52 strategic bomber took part in air military drills with South Korea in a show of force following North Korea's failed launch of a spy satellite at the end of May.

(Reporting by Jack Kim; Editing by Lisa Shumaker and Diane Craft)