The U.S. has expressed concern at an incident in which dozens of Chinese protesters surrounded the car of U.S. Ambassador Gary Locke, pelted it with objects, and blocked its entry to the American embassy in Beijing.

U.S. State Department spokesperson Victoria Nuland says Chinese authorities have expressed regret for the Tuesday incident, which occurred during a day of heated anti-Japan protests across China.

“On September 18th, approximately 50 Chinese demonstrators surrounded Ambassador Locke's official vehicle as it attempted to enter the embassy compound. They caused minor damage to the vehicle. Ambassador Locke himself was unharmed. There were Chinese security personnel standing in front of the compound. They responded and removed demonstrators from the scene, which allowed the ambassador's car to move forward.”

Locke told reporters that he was unharmed and did not feel in danger during the incident. But he said he met with Chinese Foreign Ministry officials to express concerns and to urge them to do “everything possible” to protect U.S. embassy personnel. Foreign Ministry spokesperson Hong Lei called the incident “an individual case,” but said China was investigating it.

Video of the incident posted on YouTube by dissident artist Ai Weiwei showed flag-carrying protesters shouting slogans, hurling bottles, and tearing the American flag off of the ambassador's car before a line of police moved in to surround the vehicle.

The incident came just days after a mob attack on a U.S. consulate in Libya resulted in the death of Ambassador Chris Stevens and three other diplomatic staff during protests against a U.S.-made anti-Islam video. Security has been ramped up at U.S. diplomatic offices around the world following the attacks.

Nuland said Wednesday that the Beijing incident did not appear to be related to the recent wave of anti-U.S. protests that have broken out in response to the video. She said it was likely a “target of opportunity” for demonstrators returning from a protest at the nearby Japanese Embassy.

Joseph Cheng, a political science professor at City University of Hong Kong, tells VOA he was surprised by the incident, and that Beijing likely was, as well.

“This is of course embarrassing for the Chinese authorities. I do not think that the Chinese authorities would like these events to occur,” says Cheng, who added that it was likely just a coincidence that it happened during a visit by U.S. defense chief Leon Panetta. “Naturally, any cars belonging to foreign embassies or foreign diplomats might be a target of harassment very near to the Japanese embassy.”

Thousands of Chinese took to the streets Tuesday on the sensitive anniversary of the Japanese invasion of northern China in 1931. China and Japan have been engaged in an increasingly bitter dispute over a group of islands in the East China Sea.

On Wednesday, Nuland again reiterated Washington's position that it does not take sides on the question of the sovereignty of the islands and urged China and Japan to resolve the dispute through peaceful means.

Some Chinese have questioned Washington's neutrality in the debate, since Tokyo is a strong ally of Washington and boasts a security treaty that ensures U.S. support in case of an attack on its territory.