U.S. surveillance and reconnaissance aircraft returned to an airbase in Japan's southern island of Okinawa late on Friday (May 29) after a day of surveying the area.
Information-gathering activity has been heightened after South Korea and the United States raised military alert level in the region the previous day.
On Thursday (May 28), following declarations by the communist North warning the truce ending the Korean War was dead and it was ready to attack.
North Korea ramped up tensions this week with a series of provocations rarely seen since the 1950-53 Korean War, including war threats, missile launches and a nuclear test that puts it closer to having an atomic bomb.
South Korean media reports said on Friday, Pyongyang had fired another short-range missile into the Sea of Japan.
At the Kadena airbase in Okinawa, at least one RC-135S "Cobra Ball', Royal Air Force Vickers VC10 surveillance planes as well as AWACS aircraft were seen taking off and landing regularly.
In Singapore, security was tight around the Shangri-la Hotel as official delegates from 27 governments arrived for a three-day security summit.
One of the delegates, South Korean Defence Minister Lee Sang-he, met Singapore Deputy Prime Minister and Defence Minister Teo Chee Hean ahead of the opening of the 8th annual Asia Security Summit, the Shangri-la Dialogue, which opens later on Friday.
Regional security issues, including North Korea's recent military activity will be high on the agenda. The summit is expected to run till Sunday (May 31).
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