Tensions have flashed numerous times over the Senkaku Islands, which China calls Diaoyu Islands, including face-offs between Japanese and Chinese warplanes and ships.

China says its claim to the islands extend back to 1400s, when it was used as a staging point for Chinese fisherman.

However, Japan says it saw no trace of Chinese control of the islands in an 1885 survey, so it formally recognized them as Japanese sovereign territory in 1895.

The islands were administered by the US occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.

Taiwan, which Beijing regards as a breakaway province, also claims ownership of the chain.

The most recent round of tensions was lit in 2012, when Japan nationalized the islands to ward off a planned sale to Tokyo's then-governor, a hardline nationalist apparently hoping to develop the islands.

The plan sparked massive anti-Japanese protests across China.

In 2013, China declared a formal Air Defense Identification Zone covering airspace over the islands and overlapping with airspace claimed by Japan. The ADIZ declaration required airlines flying over the waters to first notify China.

The discovery of potential oil, natural gas and methane hydrate deposits in the area has added impetus to the dispute, says Carl Schuster, a professor at Hawaii Pacific University and former director of operations at the US Pacific Command's Joint Intelligence Center.

According to a 2016 report by the Asia Maritime Transparency Initiative, China has at least 12 operational drilling rigs.

It added that Japanese defense officials fear that China might make military modifications to the rigs but concluded it was unclear whether China intends to use the rigs for civilian-military use.

The islands are also close to strategically important shipping lanes and fertile fishing grounds.

Since 2012, Chinese Coast Guard vessels and fishing boats have begun to ply the waters in growing numbers and have been accused of bullying Japan's fishermen over the last two years, says Schuster.

"Their Coast Guard vessels are larger than Japan's and, in some cases, better armed albeit with defensive weapons only," Schuster says.

What's more, he says, "a portion of China's fishing fleet is a paramilitary organization called the People's Maritime Militia. They have better communications gear than a standard fishing craft and their crews are armed and receive military training. "

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East China Sea: How an uninhabited island chain splits Japan and China - CNN