Three Chinese guard ships entered Japan’s territorial waters near the Senkaku Islands in the East China Sea on Monday, making it the 13th such intrusion this year. The intrusions come as Japan tries to bolster its naval defense amid increasing assertion by China over Southeast Asian waters.

The three vessels that intruded Japanese waters were identified as Haijing 2102, Haijing 2305 and Haijing 2350, the Japan Times reported, citing the coast guard’s 11th regional headquarters in Naha, the capital city of Okinawa Prefecture. The vessels entered the Japanese waters from the north-northwest of the islet of Kubajima at about 10 a.m. local time (9 p.m. EDT on Sunday) on Monday. The intrusion further fuels tension in the region as the Senkaku Islands controlled by Japan are also claimed by China where they are known by the name of Diaoyu Islands. 

According to a report released last month by the Japanese government, the number of scrambles by Japanese warplanes surged as high as that during the Cold War era due to growing activity by Russia and China in the region. Last month, U.S. Secretary of Defense Ash Carter also began talks with his Japanese counterpart to boost collaboration amid rising threat from China. The joint training missions between the U.S. and Japan may reportedly extend through the South China Sea to the Indian Ocean.

"We are concerned by the scope and pace of China's land reclamation activities, which are inconsistent with China's own past commitments to ASEAN countries," Carter had reportedly said in an interview to a local newspaper at the time. "We are especially concerned at the prospect of militarization of these outposts. These activities seriously increase tensions and reduce prospects for diplomatic solutions," Carter said, adding: "We urge China to limit its activities and exercise restraint to improve regional trust."

In March, Japan also began bolstering its security ties with Vietnam and the naval forces of the Philippines, along with providing maritime support to both the countries, to prepare a counter for China’s assertions in the region.