Two dozen B-1 and B-52 bombers have been put on alert
The United States is ordering extra military forces - thought to be long-range bombers - to boost defences near North Korea.
The move comes amid heightened tensions with Pyongyang, following Monday's interception of a US surveillance plane by four North Korean fighter jets in international airspace.
Washington military officials also said that the US is considering sending it own fighter jets to escort future surveillance flights.
"As part of our efforts to meet our global requirements, we are deploying additional forces to the western Pacific as US forces are preparing for possible military action elsewhere," Pentagon spokesman Lieutenant Commander Jeff Davis said.
He did not specify what forces were involved, but BBC Pentagon correspondent Nick Childs says they are understood to be long-range bombers.
Last month the United States put two dozen B-1 and B-52 bombers on alert for possible deployment to the US-held Pacific island of Guam.
The force would also be accompanied by about 2,000 personnel, the French news agency AFP reported.
Pentagon officials insist the decision was made on Friday and is not a response to Monday's air incident over the Sea of Japan.
Washington on Tuesday confirmed it would lodge a formal protest over the encounter, which follows weeks of escalating tensions with North Korea, notably over the isolated ultra-communist state's nuclear programme.
"This kind of reckless behaviour by North Korea will only lead to further
international isolation of North Korea," White House spokesman Ari Fleischer said.
North Korea has condemned the military exercises
US officials said that at one point one of the four North Korean MiG fighters involved edged within 15 metres (50 feet) of the US plane.
The escalating tensions come as the US and South Korea begin large-scale military exercises, which the North has labelled a prelude to invasion.
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.
The Pentagon said the interception happened on Sunday as its RC-135 - a model based on the Boeing 707 airliner - was on a routine intelligence mission about 150 miles (240 kilometres) off North Korea's coast.
The US 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.
US military officials said on Tuesday that one possible response to the incident would be to have fighters escort future flights.
However they say that doing so runs the risk of undercutting the US assertion
that the surveillance flights are not military threats.
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.
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
The US stopped fuel aid to North Korea, which reacted by kicking weapons inspectors out of the country and restarting its Yongbyon nuclear reactor.
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.
In recent weeks, Pyongyang has ratcheted even further its normally belligerent rhetoric.
On Sunday, Pyongyang accused the US of planning a secret strike on the Yongbyon reactor and warned such an operation could trigger "horrifying nuclear disasters".