YOKOSUKA NAVAL BASE, Japan -- A Chinese intelligence ship entered Japanese territorial waters Wednesday morning, one day after the U.S. Navy's Pacific Fleet commander reportedly discussed tensions in the East China Sea with Japanese officials.
The Chinese Dong Diao-class intelligence ship spotted near Kuchinoerabu Island, in southern Japan, is believed to have been monitoring the annual Malabar exercise, a trilateral sea drill including the United States, Japan and India, a Japan Defense Ministry spokesman said Wednesday.
The Chinese vessel "followed Indian ships that were participating in this training and traveling in Japanese waters," Defense Minister Gen Nakatani said, according to a ministry spokesman.
The Chinese ship also observed the U.S. Navy aircraft carrier USS John C. Stennis, a Japanese official told Reuters, though it was unclear whether that observation occurred in international waters.
Chinese monitoring of U.S. carriers isn't unusual. For example, a Chinese intelligence ship observed the USS George Washington during the Valiant Shield exercise in international waters off Guam in 2014.
The incursion on Wednesday came after a Chinese frigate sailed Thursday within a contiguous zone just outside the territorial waters of the Japan-administered Senkaku Islands, which China claims as its own. Japan protested the Senkaku operation by summoning China's ambassador at 2 a.m., as it occurred.
The Japan Foreign Ministry lodged a complaint Wednesday with the Chinese Embassy in Tokyo over the most recent activity.
A military vessel sailing within another nation's territorial waters conforms with international law if it conducts an "innocent passage," which precludes weapons-related operations and intelligence gathering that would harm a state's security capabilities.
However, Beijing has repeatedly protested when U.S. Navy ships in recent months have conducted innocent passages nearby Chinese-claimed artificial islands in the South China Sea. Artificial islands do not possess territorial waters under the United Nations Convention on the Law of the Sea, or UNCLOS.
On Tuesday, Pacific Fleet Adm. Scott Swift met with Nakatani in Tokyo, where the two spoke about maintaining regional stability, according to media reports.
Swift said he sees a "common theme" occurring in both the South China Sea and the East China Sea, where "a context of uncertainty and angst in the region has brought about a lack of transparency," according to an interview with the Nikkei Asian Review on Tuesday.
China's rapidly modernizing military has alarmed nations throughout the Asia-Pacific region.
Tensions over the Senkaku Islands, which the U.S. claimed after World War II and returned to Japan in 1972, have resulted in jets scrambled by both Japan and China, along with incidents of boat ramming.
China also claims about 90 percent of the South China Sea, which includes territories and waters claimed or settled by Vietnam, Malaysia, Brunei, Taiwan, the Philippines and Indonesia. China has constructed multiple artificial islands atop reefs, topping them with radar stations, artillery and runways capable of supporting large military aircraft.
It also continues to publish maps that include the "nine-dash line," a series of marks around the sea that Beijing hasn't fully explained, but has been implied as an expression of the nation's claims.
The Philippines has challenged the validity of the line at the international Permanent Court of Arbitration, which is expected to issue a ruling this month.
Although China has rejected the court's jurisdiction, U.S. and court officials have stated that as a signatory to UNCLOS, China is bound by the ruling.
U.S. officials have noticed that in advance of the ruling on the line -- which many analysts say the Philippines is likely to win -- China has been shifting its language.
"I was struck by comments that have been made, with claims outside of what the Chinese refer to as the nine-dash line," Swift said, according to the Nikkei interview. There is a new reference being made -- that I hear about traditional fishing grounds. That has raised concerns."
-- Stars and Stripes reporter Hana Kusumoto contributed to this report.