As the US focused on celebrating the Fourth of July, tensions rose briefly in the Pacific after Japan spotted Chinese and Russian warships near disputed islands in the East China Sea. Japan’s defense ministry said a Chinese frigate sailed within the “continuum zone”, just outside Japanese territorial waters around the Senkaku Islands – which Beijing also claims and calls the Diaoyu – for several minutes on Monday morning. FILE: A Japanese Coast Guard vessel and vessels, back and right, sail alongside a Japanese activist fishing boat, center with a flag, near a group of disputed islands called Diaoyu by China and Senkaku by Japan, early Sunday, August 18, 2013. (AP Photo/Emily Wang, File) That sighting came just 40 minutes after a Russian frigate had entered the waters for more than an hour, the ministry said. It was not immediately clear what was behind the latest Sino-Russian military activity in the region. Japanese defense officials have raised the possibility that the ships were there to avoid a typhoon. US ADMIRAL DECLARES SOUTH CHINA SEA ISLANDS FULLY MILITARY Deputy Chief Cabinet Secretary Seiji Kihara said Japan had lodged a protest expressing “serious concern” to Beijing over the incident. “The Senkaku Islands are an inherent part of Japan’s territory historically and under international law. The government will deal with the matter calmly but decisively to protect Japanese land, territorial waters and airspace,” Kihara said. There was no violation of territorial waters, he said, but noted that the Chinese incursion into the contiguous zone was the fourth such event since June 2016. Beijing, meanwhile, criticized Tokyo’s protest and justified the frigate’s entry as “legal and justified”. “Japan has no right to make such irresponsible remarks,” Chinese Foreign Ministry spokesman Zhao Lijian told a news conference, adding that the islands are Chinese territory. CLICK HERE TO GET THE FOX NEWS APP Japan, for its part, views China’s increasingly dynamic military activity in the East and South China seas as a threat to regional stability. Tokyo is particularly sensitive to Chinese activities near the disputed islands. The Associated Press contributed to this report.