(CNN) -- China is once again complaining that the United States violated its sovereign territory by coming near artificial islands in the South China Sea.
This time the problem occurred when a U.S. military flight strayed into disputed airspace near the islands, a senior U.S. defense official told CNN.
The United States did not intend to get within 12 nautical miles of the land, the official said, and the flight was not part of a "freedom of navigation" mission like the one that occurred in late October.
In that operation, the United States sent a warship, the USS Lassen, close to one of China's artificial islands.
That put the vessel within an area that would be considered Chinese sovereign territory -- if the United States recognized the islands as being Chinese territory, a U.S. official told CNN in October.
The Chinese Defense Ministry accused the United States of "flexing muscles" with the flight in a statement. It said the flight involved two B-52s and occurred on December 10.
"The action from the United States is a serious military provocation. It complicates and even militarizes the regional situation in the South China Sea. We demand the United States to take necessary actions to prevent such events from happening, and avoid damages to the military-to-military relations between the two countries," the ministry said.
Speaking Friday about the most recent incident involving the U.S. airplane, Bill Urban, a U.S. Defense Department spokesman, said the flight was not meant to provoke the Chinese."There was no intention of flying within 12 nautical miles of any feature," he said in a statement. "This was not a freedom of navigation operation."
He said the United States routinely conducts B-52 training missions over the South China Sea.
"These missions are designed to maintain readiness and demonstrate our commitment to fly, sail and operate anywhere allowed under international law," he said.
Tensions have ratcheted up since China reclaimed some 2,000 acres of land in a massive dredging operation, turning sandbars into islands equipped with airfields, ports and lighthouses. The work started in 2014 on three main reefs in the Spratly Islands -- Fiery Cross, Subi and Mischief Reef.
The new islands created friction with the United States, which sails and flies its assets in the vicinity of the reclaimed islands, citing international law and freedom of movement.
Last May, the United States flew over the islands, triggering warnings from the Chinese navy. In October, the Lassen incident occurred.
When China complained, a Defense Department official told CNN: "We will fly, sail and operate anywhere in the world that international law allows. U.S. Freedom of Navigation operations are global in scope and executed against a wide range of excessive maritime claims, irrespective of the coastal state advancing the excessive claim."
China's neighbors complained about the islands, too. Brunei, Malaysia, the Philippines, Taiwan and Vietnam all dispute sovereignty of several island chains and nearby waters in the South China Sea.
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