In a signal of deepening military cooperation between India, the United States and Japan, the three countries have deployed some of their largest warships and submarines in the Indian Ocean for an annual naval exercise that is conducted with an eye on China.

The naval drills have expanded in the last two years amid growing concerns over Chinese maritime assertiveness not just in South China Sea, but also in the Indian Ocean.

The Malabar exercises are the most visible symbol of New Delhi’s strengthening security ties with the United States, which were reaffirmed last month by Indian Prime Minister Narendra Modi and U.S. President Donald Trump.

Exercise expands

This year’s weeklong maneuvers on the high seas, which began Monday, involve more than 15 warships, including the US nuclear powered aircraft carrier, USS Nimitz, India's aircraft carrier INS Vikramaditya, and Japan’s largest warship, the JS Izumo.

The focus this year is on anti-submarine warfare.

“The exercise has “grown in scope and complexity to address the variety of threats to maritime security in the Indo-Asia Pacific,” according to a US embassy statement.

In recent months, the Indian Navy has recorded an unusual surge in the number of Chinese naval vessels in the Indian Ocean and tracked at least seven Chinese submarines entering the region since December 2013 according to military observers. They believe this could be muscle flexing by Beijing.

“We understand that there are about 13 vessels of different kind, whether for anti-piracy or for surveillance, are currently in the Indian Ocean,” says Vijay Sakhuja, Director of the National Maritime Foundation in New Delhi. “So it certainly is like the Chinese navy is in your backyard and it is a matter of concern.”

Chinese funding and assistance for building ports in Pakistan and Sri Lanka has added to Indian concerns about the forays by Chinese ships.

FILE - A Japan Maritime Self-Defense Force technic FILE - A Japan Maritime Self-Defense Force technician enters the water during a 2016 exercise with the Indian, Japanese and U.S. navies in the Indo-Asia-Pacific. FILE - A Japan Maritime Self-Defense Force technician enters the water during a 2016 exercise with the Indian, Japanese and U.S. navies in the Indo-Asia-Pacific.

Beijing wary

Beijing on its part remains suspicious of the trilateral naval engagement, particularly after it expanded to include Japan since 2015, believing that it is an effort to contain its influence.

Ahead of the Malabar exercises this year, Chinese foreign ministry spokesman Geng Shuang said that while China had no objection to normal cooperation between countries, “We hope that this kind of relationship and cooperation will not be directed against third country and that it will be conducive to the regional peace and security."

Sakhuja says the three countries are developing “a coordinated approach, to not contain, not even counter, just to be around in the Indian Ocean to just watch how the Chinese navy would be unfolding itself in the coming years.”

FILE - Team members from the Indian and U.S. navie FILE - Team members from the Indian and U.S. navies take part in a joint training exercise aboard an Indian ship in June 2016. FILE - Team members from the Indian and U.S. navies take part in a joint training exercise aboard an Indian ship in June 2016.

During Prime Minister Modi’s visit to Washington last month, Trump called their security partnership “incredibly important” and both countries pledged to expand maritime security cooperation. India has also come increasingly close to Japan in the last two years – during a visit to Tokyo in May. Indian Defense Minister Arun Jaitley said that India is looking to strengthen military cooperation with Japan.

But New Delhi turned down a request by Australia to join the trilateral exercise.

The ships of the three nations streamed into the high seas as a tense standoff between India and China showed no signs of easing in the high Himalaya mountains.

Soldiers from the two countries have been confronting each other since last month, when Indian soldiers obstructed a Chinese road-building project in a plateau disputed between China and Bhutan, a close ally of India. China has repeatedly called on India to withdraw its troops, but so far both sides have refused to back down.