SEOUL (Reuters) - North Korea fired a ballistic missile from a submarine on Saturday, but it appears to have failed soon after launch, South Korea’s military said.

The launch comes at the end of a week of sharply rising tensions on the Korean Peninsula, and only a day after the United States and South Korea pledged to deploy an advanced anti-missile system to counter threats from Pyongyang

North Korea on Thursday warned it was planning its toughest response to what it deemed a “declaration of war” by the United States. The warning followed Washington’s blacklisting of the isolated state’s leader Kim Jong Un for alleged human rights abuses.

South Korea’s Office of the Joint Chiefs of Staff said in a statement that the missile was launched at about 11:30 a.m. Seoul time (0230 GMT) in waters east of the Korean Peninsula.

The missile was likely fired from a submarine as planned, but appears to have failed in the early stage of flight, the Joint Chiefs said.

Japan, the United States and South Korea’s military condemned the missile launch as a flagrant violation of United Nations sanctions.

South Korea’s Yonhap news agency said the missile’s engine successfully ignited, but the projectile soon exploded in mid-air at a height of about 10 km (6 miles), and covered not more than a few kilometers across the water.

The South’s military declined to confirm those details citing its policy of not publicly commenting on intelligence matters.

The missile was detected in the sea, southeast of the North Korean city of Sinpo, South Korea’s military said. Satellite images indicate Pyongyang is actively trying to develop its submarine-launched ballistic missile program in this area, according to experts.

The U.S. Strategic Command, whose mission is to detect and prevent strategic attacks against the United States and its allies, said it had detected what it believed was a KN-11 submarine-launched ballistic missile (SLBM).

It was fired near Sinpo, the North’s east coast port, and then fell into the sea between there and Japan, the command said in a statement.

JAPAN CONDEMNS LAUNCH

Reclusive North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in an armistice, not a peace treaty. The North regularly threatens to destroy Japan, South Korea and the South’s main ally, the United States.

The missile launch is a “clear challenge to U.N. Security Council resolutions,” Japanese Prime Minister Shinzo Abe said on Saturday, according to the Kyodo news agency.

“We should strongly condemn the launch by working with the international community,” Abe told reporters, but said the launch did not gravely affect Japan’s national security.

The United States said it was monitoring and assessing the situation in close coordination with its regional allies and partners.

“We strongly condemn North Korea’s missile test in violation of UN Security Council Resolutions, which explicitly prohibit North Korea’s use of ballistic missile technology,” said Gabrielle Price, spokeswoman for the Bureau of East Asian and Pacific Affairs at the U.S. State Department.

Slideshow ( 4 images )

“These actions, and North Korea’s continued pursuit of ballistic missile and nuclear weapons capabilities, pose a significant threat to the United States, our allies, and to the stability of the greater Asia-Pacific,” she added.

The North has conducted a string of military tests that began in January with its fourth nuclear test and included the launch of a long-range rocket the following month.

The U.N. Security Council imposed harsh new sanctions on the country in March for its nuclear test and rocket launch.

North Korea rejects the sanctions as infringement of its sovereignty and its right to space exploration.

Slideshow ( 4 images )

In late June, North Korea launched what appeared to be an intermediate-range missile to a high altitude before it plunged into the sea after covering 400 km in the direction of Japan, South Korean military officials said. That was widely seen as a technological advance for the North after several test failures.

South Korea and the United States said on Friday they would deploy the Terminal High Altitude Area Defense (THAAD) anti-missile system with the U.S. military in South Korea to counter the threat from nuclear-armed North Korea, drawing a sharp and swift protest from neighboring China, Pyongyang’s sole major ally.

China’s Foreign Minister Wang Yi said on Saturday that THAAD exceeded the security needs of the Korean Peninsula.

“We have every reason, and the right, to question the real conspiracy behind this move,” Wang was quoted by state news agency Xinhua as saying during a trip to Sri Lanka.

Pyongyang last conducted a test of a submarine-launched ballistic missile in April, calling it a “great success” that provided “one more means for powerful nuclear attack,” although it had not had a successful SLBM test flight.

Speaking to reporters on the sidelines of the NATO summit in Warsaw, U.S. Army General Curtis Scaparrotti, the top NATO commander and former commander of U.S. forces in South Korea, said he worried about North Korea’s potential to hit the continental United States with a ballistic missile.

“Kim Jong Un and his regime continue to test and work on their ballistic missile capability, and with every launch they’re getting better and they’re working out their problems,” Scaparrotti said. “It’s a serious threat.”

A report on 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University, said that “even with frequent testing, North Korea’s SLBM program still faces significant technical challenges and will likely require several years to deliver an operational system.”