Washington has accused North Korea of "reckless behaviour" after four of its fighter jets intercepted a US reconnaissance plane in international airspace.
The Pentagon said the RC-135 plane was on a routine mission
White House spokesman Ari Fleischer said Pyongyang was testing the tolerance of the US and the incident could lead to further international isolation of the country.
Mr Fleischer confirmed that the US would lodge a formal protest over Sunday's incident when the North Korean jets shadowed the plane for 22 minutes.
US officials said that at one point one MiG fighter edged within 15 metres (50 feet) of the US plane.
This kind of reckless behaviour by North Korea will only lead to further international isolation of North Korea
Ari Fleischer, White House spokesman
News of the incident came as the US and South Korea began large-scale military exercises and amid continuing tension over the North's nuclear programme.
The Pentagon said the interception happened on Sunday as its RC-135 plane was on a routine intelligence mission about 150 miles (240 kilometres) off North Korea's coast.
The Pentagon said initially that one of the North Korean jets involved had locked its radar weapons system onto the US plane, the final step before firing a missile.
However, it later said that the aircraft had merely been "acquired" by the MiG's radar.
The US plane broke off its mission and returned to its base in Kadena, Japan, Pentagon spokesman Lieutenant Commander Jeff Davis said.
He said it was the first such incident since August 1969, when North Korean jets shot down a US reconnaissance plane, killing 31 people aboard.
The RC-135 is a converted Boeing 707 packed with electronic receivers as well as equipment for the long-range photography of ballistic missile tests.
The US says its joint military exercises with the South are a planned annual event and not connected to friction with the North.
Codenamed Foal Eagle, they involve 5,000 American soldiers.
But the North has claimed the exercises herald an attack, and in recent days has ratcheted up its even normally belligerent rhetoric.
On Sunday, Pyongyang accused the US of planning a secret strike on its recently re-started Yongbyon nuclear reactor and warned such an operation could trigger "horrifying nuclear disasters".
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
North Korea has been embroiled in a tense stand-off with the US since news broke in October of a secret North Korean nuclear programme.
The US stopped fuel aid to North Korea, which reacted by kicking weapons inspectors out of the country and restarting 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.
Monday's aerial drama echoed the April 2001 collision between a US EP-3 surveillance plane and a Chinese fighter jet in international air space over the South China Sea.
The damaged US plane was obliged to land at a Chinese base, and its was crew detained for 11 days.
The Chinese pilot was lost, presumed dead.