Tokyo denounced Beijing on Thursday after a Chinese frigate entered a contiguous zone just outside Japanese territorial waters earlier in the day near the disputed Senkaku Islands in the East China Sea.

The move comes amid mounting international pressure on China over its growing maritime assertiveness.

“Beijing’s action of sending a military ship for the first time would further escalate tensions unilaterally and we are gravely concerned about it,” Chief Cabinet Secretary Yoshihide Suga told a news conference.

Suga also emphasized Japan’s sovereignty over the islands, which are administered by Japan but claimed as Diaoyu by China and Tiaoyutai by Taiwan, saying that Tokyo will work with the United States in demanding that China stop taking actions that raise tensions.

According to the Defense Ministry, the Maritime Self-Defense Force destroyer Setogiri confirmed that a Jiangkai I class frigate entered contiguous waters northeast of Kubajima, one of the Senkaku chain’s five primary islets, at around 12:50 a.m. Thursday.

The ship then left the zone from an area near Taishojima, the smallest of the main islets, at around 3:10 a.m. and headed north, the ministry said, adding that it did not enter territorial waters.

Since September 2012, when Tokyo effectively nationalized the chain, Beijing has been sending coast guard ships close to the contiguous waters to assert its sovereignty over the Senkakus. But Thursday was the first time it had sent a naval ship into the waters.

In Tokyo, Vice Foreign Minister Akitaka Saiki summoned Chinese Ambassador to Japan Cheng Yonghua to the ministry to lodge a protest right after the Chinese ship was spotted.

Kyodo News reported that the Chinese Defense Ministry defied Tokyo’s protest and said it is legal for its military to operate in waters under its jurisdiction, emphasizing that the Senkaku Islands are part of Chinese territory.

The contiguous waters comprise a zone 12 nautical miles outside of territorial waters that stretch for 12 nautical miles from the coastline. Entering the contiguous waters is not a violation of international law, because the area is essentially the high seas, where international ships can sail.

In fact, Japan did not lodge a protest with Russia when Russian naval ships entered the same area the previous night. According to the ministry, the MSDF destroyer Hatakaze spotted three Russian vessels at around 9:50 p.m. Wednesday. The vessels left the zone at around 3:05 a.m. Thursday.

Suga said Tokyo saw no problem with Russia’s action and that it protested to Beijing only because it has been continually issuing warnings over the issue. In addition, Russia does not claim sovereignty over the Senkakus.

The action comes at a time when Japan and the U.S. are pressuring China to comply with international law while an arbitration court in The Hague prepares to issue a verdict on a territorial dispute in the South China Sea between Beijing and Manila soon.

At the Group of Seven summit in Mie Prefecture last month, Prime Minister Shinzo Abe proposed three principles that call on countries to comply with international law and not change the status quo by force when making territorial claims.

At the recent Shanri-La Dialogue in Singapore, U.S. Defense Secretary Ash Carter warned that China was “erecting a Great Wall of self-isolation” through its land reclamation activities in the South China Sea. Japan is scheduled to take part in a joint trilateral naval exercise with the United States and India starting Friday near the Senkaku Islands.

Retired Vice Adm. Yoji Koda of the MSDF said China is trying to shift the global focus to the East China Sea to dodge criticism of its activities in the South China Sea.

“Beijing will continue its activities in the East China Sea until the verdict (on the South China Sea), which is likely to go against China,” said Koda. “It’s better to re-direct the global focus to the East China Sea where there is no lawsuit filed against it.”

Bonji Ohara, a research fellow at the Tokyo Foundation, said that China’s provocations are likely to continue but said it is unclear whether the incident is a direct response to pressure from Japan and the U.S.