Beijing ( CNN ) -- 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 . Chinese outrage over islands troubles Japanese public . `` 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 . 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 . Dangerous Rocks : Can both sides back off peacefully ? CNN 's Steven Jiang reported from Beijing ; Jethro Mullen from Hong Kong . CY Xu and Stan Grant also contributed to this report .
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 .