China acknowledged Wednesday it let two American B-52 bombers fly unhindered through its newly declared air defense zone in the East China Sea despite its earlier threat to take defensive measures against unidentified foreign aircraft.

The U.S. flights, which tested the Chinese zone for the first time since it was declared over the weekend, raised questions about Beijing’s determination to enforce its requirement that foreign aircraft identify themselves and accept Chinese instructions.

China’s lack of any action suggested that it was merely playing out a diplomatic game to establish ownership over the area rather than provoke an international incident.

The flights followed days of angry rhetoric and accusations over Beijing’s move, designed to assert Chinese claims to a group of uninhabited islands controlled by Japan.

The U.S. and Japan have said they don’t acknowledge the zone, and Taiwan and South Korea, both close to the U.S., also rejected it.

Vice President Joe Biden will raise objections over the Chinese declaration when he visits the country next week, senior administration officials said in Washington. They said Biden would press Beijing policymakers on the “unsettling” pattern of how they deal with neighbors.

A Chinese Defense Ministry statement said the U.S. planes were detected and monitored as they flew through the area for two hours and 22 minutes. It said all aircraft flying through the zone would be monitored and that “China has the capability to exercise effective control over the relevant airspace.”

Asked repeatedly about the incident at a regularly scheduled briefing, Foreign Ministry spokesman Qin Gang said it had been handled according to procedures laid out in the Saturday statement but offered no specifics.

“Different situations will be dealt with according to that statement,” Qin said.

The U.S., which has hundreds of military aircraft based in the region, described the flights as a training mission unrelated to China’s announcement of the zone. U.S. officials said the two unarmed B-52 bombers took off from their home base in Guam around midday and were in the zone that encompasses the disputed islands for less than an hour before returning to their base, adding the aircraft encountered no problems.

The bomber flights came after State Department spokeswoman Jen Psaki said China’s move appeared to be an attempt to change the status quo in the East China Sea.

“This will raise regional tensions and increase the risk of miscalculation, confrontation and accidents,” she told reporters.

The Pentagon said Defense Secretary Chuck Hagel discussed the situation by telephone Wednesday with his Japanese counterpart, Itsunori Onodera, and Hagel said U.S. military operations “will not in any way change as a result of China’s announcement.”

Australia, meanwhile, said it called in the Chinese ambassador to express concern about the sudden zone declaration.

“The timing and the manner of China’s announcement are unhelpful in light of current regional tensions, and will not contribute to regional stability,” Australian Foreign Minister Julie Bishop said in a statement.

Beijing’s move fits a pattern of putting teeth behind its territorial claims and is seen as potentially leading to dangerous encounters depending on how vigorously China enforces it — and how cautious it is when intercepting aircraft from Japan, the U.S. and other countries.