The US Air Force has resumed reconnaissance flights in international airspace off North Korea, 11 days after a US plane was intercepted by North Korean fighters jets.
The Pentagon said its spy plane was on a routine mission
"Those flights have recommenced," said Admiral Thomas Fargo, the commander of US forces in the Pacific.
He added that "prudent measures" had been put in place to protect the flights.
The latest flight was not escorted by fighters but was monitored closely by airborne and ship radars, other US officials, speaking anonymously, told the Associated Press news agency.
The missions were suspended after the 2 March incident when four North Korean planes intercepted an unarmed and unescorted RC-135S aircraft over the Sea of Japan, about 150 miles (240 kilometres) off North Korea's coast.
That was the first such incident since 1969, and was seen by the US as a provocative action.
On Tuesday, the Pentagon said that that the fighters appeared to be trying to force the US plane to land in North Korea.
The announcement came hours after Washington warned that North Korea's nuclear programme may be much more advanced than previously thought.
Map shows range of Taepodong 1 missile, flown over Japan in 1998. Range 1,500-2,000 km, payload: 1,000 kg
Evidence that North Korea preparing flight test of Taepodong 2. Range up to 8,000 km (could reach western US)
Other missiles: Scud-B: Range 300 km, payload 1,000 kg
Scud-C: Range 500 km, payload 7600-800 kg
Scud-D (Nodong): Range 1,000-1,300 km, payload: 700-1,000 kg
Assistant Secretary of State James Kelly said that Pyongyang's uranium enrichment programme could be only months away from producing weapons-grade material.
North Korea has already re-started its Yongbyon nuclear reactor, which could soon be reprocessing nuclear fuel into plutonium - another way of making nuclear bombs.
The BBC's State Department correspondent Jon Leyne says experts believe the North could soon have a production line able to make up to a bomb a month.
On Thursday, Japanese newspapers reported that Pyongyang might be preparing to test-fire its Rodong ballistic missile, which has a range long enough to reach almost anywhere in Japan.
The Japanese Government played down the reports, saying it had "no specific information" on a possible launch.
Japan's defence agency confirmed that it had sent a surveillance warship closer to the Korean Peninsula, but insisted the deployment was part of regular patrol activities.
No direct talks
Mr Kelly confronted the North Koreans over the existence of a nuclear programme last October, the start of the latest crisis.
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
March 2: N Korean jets intercept US surveillance plane in international airspace
10 March: N Korea fires second missile into sea
On Wednesday, he admitted that most of Washington's allies in Asia were pressing for the US to acquiesce to Pyongyang's demands for direct talks with the North.
But Washington is sticking to its policy of pushing for multilateral talks, saying North Korea's nuclear ambitions concern the rest of the world, not just the US.
Mr Kelly there was little hope the North would meet US nuclear disarmament demands.
"There is not the slightest sign they [North Korea] have any interest in stopping," he said.
Meanwhile, South Korean President Roh Moo-Hyun has again called for "urgent" steps to resolve the nuclear crisis.
Mr Roh said on Thursday that resolving the standoff was his top priority, as another war on the Korean peninsula would reduce South Korea's prosperity to ashes in a moment.