North Korean fighter jets have intercepted a US reconnaissance plane in international air space and shadowed it for 22 minutes, a Pentagon spokesman has said.
The Pentagon said the RC-135 plane was on a routine mission
The spokesman said four North Korean MiGs at one point approached the RC-135 plane as close as 15 metres (50 feet) during the incident, which took place early on Sunday.
"The fighters were armed. During the intercept at least one of the fighters engaged fire support radar and locked on to the RC-135," said Lieutenant Commander Jeff Davis.
He said the US plane had to break off its mission. It returned to its base in Kadena, Japan, without damage.
The BBC's Nick Childs at the Pentagon says it is the first such incident in 30 years, and it comes against the background of the continuing US-North Korean tensions over Pyongyang's nuclear ambitions.
The incident happened as the US and South Korea were due to begin large-scale joint military exercises on Tuesday.
US officials said they would issue a formal protest over the incident, which happened at about 0840 local time (0140 GMT) about 240 kilometres (150 miles) off the North Korean coast in the Sea of Japan, also known as the East Sea.
Commander Davis said the manned RC-135, a converted Boeing 707 plane, was on a routine intelligence mission when it was approached by two advanced MiG29s and two other jets, believed to be MiG23s.
The spokesman said he had no indication that US fighter jets were called to protect the reconnaissance aircraft before it returned to Kadena.
He said the "lock-on" manoeuvre would usually be an action indicating a possible intent to fire, but added that the North Korean planes did not act aggressively.
The spokesman said it was the first such incident since August 1969, when North Korean jets shot down a US reconnaissance plane over the Sea of Japan, killing 31 people aboard.
The incident came as the US and South Korea are due to begin joint military exercises, designed to improve Seoul's defence capabilities against " external aggression".
16 Oct: US says N Korea admits to a secret nuclear programme
14 Nov: US halts oil shipments to N Korea
22 Dec: N Korea removes monitoring devices at Yongbyon nuclear plant
31 Dec: UN nuclear inspectors forced to leave
10 Jan: N Korea pulls out of anti-nuclear treaty
12 Feb: IAEA refers issue to UN Security Council
27 Feb: US says Yongbyon reactor restarted
US officials have said that the drills, codenamed Foal Eagle and involving a total of 5,000 American soldiers, are a planned annual event, unrelated to the stand-off between the US and Pyongyang over North Korea's nuclear ambitions.
But the North has already warned the US against any military build-up in the region.
Last Wednesday Pyongyang issued a statement saying that the forthcoming exercises could herald an attack.
North Korea has been embroiled in a tense stand-off with the US since American officials said last October that Pyongyang had admitted to having a secret nuclear programme.
The US stopped fuel aid to North Korea, which reacted by kicking weapons inspectors out of the country and re-starting its nuclear plant at Yongbyon.
It also pulled out the nuclear Non-Proliferation Treaty, which seeks to control the spread of nuclear arms.
North Korea wants direct talks with Washington to resolve the issue, but the US has said it will not give in to what it sees as North Korean blackmail.