Six Chinese patrol boats entered territorial waters near the disputed Senkaku or Diaoyu islands today, escalating tensions in an ongoing standoff between China and Japan.

China's Foreign Ministry said the ships were dispatched to conduct maritime surveillance and a mission of "law enforcement over its maritime rights." The boats were ordered to leave, but operators on board one of the Chinese ships said, "Diaoyu is China's territory," according to the Japanese Coast Guard.

"We regret this intrusion," chief government spokesman Osamu Fujimura said. "We have urged the Chinese to leave immediately."

Relations between Asia's two biggest economies have plunged to new lows, since Japan purchased the uninhabited rocky islets for $26 million earlier this week in the face of Chinese protests. Tokyo has said the move was meant to prevent any development on the island, known as Senkaku in Japan, and ease tensions as a result.

China has urged Japan to revoke the purchase immediately, saying it violates Chinese sovereignty.

The ongoing dispute has prompted large-scale anti-Japanese protests throughout China, and threatens to affect trade and tourism in both countries.

An executive with Nissan said Sino-Japanese tensions had forced the company to pare marketing events in China, while some Chinese travel companies have stopped selling tour packages to Japan altogether.

A spokesman with the Japan National Tourism Organization said the number of Chinese visitors dropped nearly 20% when similar tensions flared up in 2010, but the immediate impact of the latest dispute was unclear.

Meanwhile, the Japanese Consulate General in Shanghai said at least four people have been injured as a result of anti-Japanese backlash. In one incident, a Japanese man had hot noodles thrown in his face. In another incident, a man was kicked several times after being asked whether he was Japanese, according to a statement posted by the Consulate General online.

On Thursday, a man in Shanghai's Baoshin District reportedly set his Honda Civic on fire, in front of a Honda dealership, unfurling signs that read "Japanese devils return home."

In Japan, the city of Kobe scrapped a welcoming ceremony for 3,000 Chinese visitors, at the request of the tour organizer, according to the Kyodo News. At least two concerts planned to mark the 40 th anniversary of the normalization of Japan-China diplomatic ties have been cancelled in Beijing and Tokyo.