The US has advised its commercial airlines to take all necessary safety precautions when flying over a group of small, disputed islands in the East China Sea.
The announcement came amid a diplomatic row over China's newly declared air defence zone there.
China demanded over the weekend that aircraft submit flight plans to Chinese officials when traversing the zone, raising the stakes in a territorial standoff between Beijing and Tokyo, which both claim the contested islands.
"We're attempting to determine whether the new rules apply to civil aviation, commercial air flight," Jen Psaki, US State Department spokesperson, said on Wednesday.
"In the meantime, US air carriers are being advised to take all steps they consider necessary to operate safely in the East China Sea region."
Psaki said US officials were still evaluating whether the commercial airlines must advise Chinese authorities when passing over the islands.
Meanwhile, the South Korean military said on Thursday that one of its planes had flown through the zone without China's knowledge.
"We did not notify China," a South Korean Defence Ministry spokesman said.
The flight on Tuesday was part of a regular military surveillance exercise around a disputed, submerged, South Korean-controlled rock known as Ieodo that has been another source of diplomatic tension with China.
The spokesman said South Korean military planes would continue to fly over the Ieodo area without telling China.
The US also defied Chinese demands, flying two unarmed B-52 bombers over the islands on Tuesday without informing Chinese authorities.
China's Defence Ministry said it had monitored the US bombers, while a Pentagon spokesman said the planes had not been observed or contacted by Chinese aircraft.
The US indicated that more military flights into the Chinese defence zone can be expected, and came out in support of Japan in the dispute.
Chuck Hagel, US defence secretary, told his Japanese counterpart in a telephone call on Wednesday that the two nations' defence treaty covers the disputed small island group.
Hagel "commended the Japanese government for exercising appropriate restraint", a Pentagon spokesman said.
Some experts say the Chinese move was aimed at eroding Japan's claim to administrative control over the area, including the tiny uninhabited islands known as the Senkaku in Japan and the Diaoyu in China.
The US does not take a position on the sovereignty of the islands but recognises that Japan has administrative control over them and the US is therefore bound to defend Japan in the event of an armed conflict.
US Vice President Joe Biden is due to visit China, Japan and South Korea during a week-long trip and will seek to ease the ongoing dispute, senior US administration officials said.
"The visit to China creates an opportunity for the vice president to discuss directly with policy makers in Beijing this issue to convey our concerns directly and to seek clarity regarding Chinese intentions," a senior administration official said.
"It also allows the vice president to make the broader point that there's an emerging pattern of behaviour that is unsettling to China's own neighbours."
Biden will not be making a demand on a specific issue, but rather will raise the topic as part of talks spanning a range of themes, the official said.
Source: Al Jazeera and agencies