Story highlights The U.S. says it has expressed concern to the Chinese authorities

China says it is investigating what it describes as an "isolated case"

About 50 protesters gathered around the car, some throwing objects

The incident took place amid anti-Japanese protests in China

The Chinese authorities are investigating an incident in which dozens of angry protesters surrounded the car of U.S. Ambassador Gary Locke in a Beijing street, some throwing objects at the vehicle before security forces intervened to protect it.

The United States has expressed concern to China about the unusual disturbance on Tuesday, which comes at a sensitive time for American diplomats following the death of Ambassador Christopher Stevens in an attack in Libya last week.

The harassment of Locke's black Cadillac took place as anti-Japanese demonstrators marched in numerous Chinese cities amid tensions between the two Asian countries over a disputed group of islands, known as Senkaku in Japan and Diaoyu in China.

"Our mission is located very close to the Japanese mission, and there were some chantings of slogans with regard to the Senkaku Islands," Victoria Nuland, a State Department spokeswoman, said Wednesday at a regular news conference . "So our preliminary assessment is that it was related to that issue and not to the issues in the Middle East."

She said China has expressed "regret" over the incident, which caused "minor damage" to the car and left Locke unharmed.

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A video of the events uploaded Wednesday to the dissident artist Ai WeiWei's channel on YouTube shows about 50 protesters, some of them carrying Chinese flags, gathering around Locke's car as it tries to enter the embassy compound.

A few of the demonstrators are seen hurling objects at the front of the vehicle before uniformed security officers manage to shield the car, enabling it to drive a short distance up the street and enter the compound through a different gate.

The incident was an "isolated case," Hong Lei, a Chinese foreign ministry spokesman, said Wednesday at a regular news briefing in Beijing.

"Relevant Chinese authorities are investigating and will handle the result properly," he said.

The long-running dispute between China and Japan over the islands -- situated between Okinawa and Taiwan in the East China Sea -- has escalated in the past two weeks after the Japanese government announced the acquisition of several of the islands from a private Japanese owner.

China declared the purchase illegal and has sent patrol vessels into the waters around the islands, drawing protest from the Japanese government.

The Chinese authorities also allowed the anti-Japanese protests to take place over several days. Some of the demonstrations turned violent, causing damage to Japanese businesses and prompting some companies to temporarily suspend operations.

The United States, a key ally of Japan, has refused to take sides on the competing sovereignty claims over the islands.

U.S. Defense Secretary Leon Panetta, who met with senior officials in both Tokyo and Bejing in recent days, has urged the two countries to resolve the issue through peaceful negotiations.