North Korean fighter jets threatened an unarmed U.S. reconnaissance aircraft over the East Sea/Sea of Japan on Sunday, increasing tensions about Pyongyang's nuclear arms and missile programs.
Two North Korean MiG-29s and two other jets, probably MiG-23s, armed with air-to-air missiles shadowed the Air Force RC-135 jet in international airspace before locking on the U.S. jet with targeting radar, said Navy Lt. Cmdr. Jeff Davis, a Pentagon spokesman.
The locked radar illumination is a step in preparing to fire a missile at the aircraft.
At the White House, a senior administration official said the incident was a "provocation" that will not be ignored. A formal protest to the 20-minute encounter is planned, the official said.
"The United States is in close consultation with South Korea and other allies on this incident, and we are consulting on how we will protest this and the most appropriate way," the senior official said.
"This is the kind of behavior that further isolates North Korea from the rest of the international community."
It was the first time North Korean fighter jets conducted an intercept of a U.S. intelligence aircraft since 1969. In that year, North Korea shot down an EC-121 electronic intelligence-gathering plane, killing 31 Americans.
Cmdr. Davis declined to characterize the encounter but said the RC-135 mission, known as a Cobra Ball intelligence-gathering mission, was routine. The militarized Boeing 707 jet was operating 150 miles from North Korea's east coast over the East Sea/Sea of Japan.
Other administration officials said the intercept was a deliberate provocation by North Korea, which appears to be escalating tensions with the United States.
"At this point, the North Koreans are going through the checklist of things we expect them to do," said a senior military official. "At some point they will run out of things on their provocation checklist and start breaking new ground."
U.S. officials are concerned that North Korea will miscalculate and undertake some action that could lead to a war.
"The closest point of approach was about 50 feet at equal altitude," Cmdr. Davis said. "The fighters were armed, and during the intercept they acquired the RC-135 on their [target] acquisition radar."
The RC-135 proceeded with its mission and returned to its base at Kadena, Japan, he said.
"We continue to fly legal reconnaissance and surveillance in international airspace in a variety of locations around the world, and we retain the right to continue to conduct them," Cmdr. Davis said.
Last week, a North Korean MiG-23 flew near South Korea off North Korea's western coast. Officials said that did not appear to be a provocation like the incident Sunday morning.
U.S. intelligence officials said regular RC-135 flights have been conducted on an almost daily basis and were successful in detecting the restart of North Korea's nuclear reactor at Yongbyon last week.
Tensions have been rising since October, when North Korea admitted to having a secret uranium-based nuclear-weapons program. North Korea then announced that it is withdrawing from the nuclear Non-Proliferation Treaty and has restarted a small nuclear reactor that can be used to produce plutonium for nuclear bombs.
Sunday's aerial encounter follows North Korea's flight testing last week of a new long-range anti-ship cruise missile. The test coincided with the inauguration of South Korea's new president.
North Korea has also stepped up its rhetoric against the United States in recent days.
Pyongyang's official communist media outlets have denounced planned U.S.-South Korean military maneuvers as preparation for war and stated that any conflict on the peninsula would result in a nuclear war.
A U.S. EP-3 intelligence-gathering aircraft was nearly destroyed by a Chinese F-8 interceptor jet that flew too close to the American aircraft during an incident April 1, 2001.
The Chinese jet crashed into the South China Sea, killing its pilot. The U.S. aircraft was damaged and made an emergency landing at a Chinese military base on Hainan island.
The Chinese military held the 23 American crew members prisoner for 11 days and forced the United States to cut up the aircraft and ship it out of the country.
On Friday, North Korea's official radio said in a broadcast monitored by the U.S. government that "U.S. imperialists insanely carried out aerial espionage activities against our republic in February."