North Korea on Tuesday warned that joint military exercises between South Korea and the United States are pushing the situation on the Korean Peninsula “to the brink of a war” and asked for an urgent meeting of the U.N. Security Council.

In a letter to the council president, North Korea’s U.N. Ambassador Jan Song Nam accused the United States of “creating the danger of war” with the annual drills which began Monday.

North Korea has threatened nuclear strikes in response to the exercises which it calls an invasion rehearsal.

This kind of fiery rhetoric by Pyongyang is not unusual but the warning comes at a time of more tension following the defection of a senior North Korean diplomat and a U.S. plan to place a high-tech missile defence system in South Korea.

South Korea’s Joint Chiefs of Staff later said North Korea had fired a ballistic missile from a submarine off its east coast into the sea on Wednesday morning. South Korean defence officials said it wasn’t immediately known whether the launch was successful or not.

Jan said in the letter that the situation on the Korean Peninsula “has become unprecedentedly instable (cq) due to the introduction of nuclear strategic bombers, anti-ballistic missile system of Terminal High Altitude Area Defence (THAAD) and other strategic assets by the U.S. to the Korean Peninsula and its vicinity.”

He said the Security Council had previously “unjustifiably ignored” several requests by the Democratic People’s Republic of Korea — the country’s official name — to put the U.S.-South Korean exercises on its agenda.

The ambassador warned that if the council again ignored the North’s request to discuss the exercises, it will not only give up its responsibility for maintaining international peace and security but become “a political tool” of the United States.

“It is the consistent stand of the DPRK government to fundamentally terminate the danger of a nuclear war posed by the U.S. by dint of the powerful nuclear deterrence and defend the regional and global peace,” Jan said.

He also warned that “peace, security and relaxation of tension on the Korean Peninsula and in the region cannot be thinkable unless the U.S.-South Korea aggressive joint military exercises are totally halted.”

Kiyoshi Odawara, a senior Japanese Foreign Ministry official, told a U.N. Security Council meeting Tuesday on halting the spread of weapons of mass destruction that North Korea’s nuclear test in January and ballistic missile launches — including one on Aug. 3 in which a rocket landed in Japanese waters — pose “clear challenges to the global nonproliferation regime and cannot be condoned for any reason.”

Odawara strongly urged North Korea to refrain from further provocations and to comply with council resolutions that call for an end to its nuclear weapons program. “These efforts are crucial, and their significance and impact cannot be underestimated,” he said.

China’s U.N. Ambassador Liu Jieyi told the council that “nonproliferation cannot be used as a pretext to beef up military deployment, step up military presence and scale up military exercises.”

“All parties should stay consistently engaged in the process towards the denuclearization of the Korean Peninsula, stay committed to the peace and stability of the peninsula … and avoid any action that is provocative to each other and may escalate tensions,” Liu said.

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Associated Press writer Dave Bryan contributed to this report from the United Nations