Japan lodged a protest against Beijing today, after a Chinese government airplane was spotted near the disputed Senkaku Islands, prompting Tokyo to scramble F-15 fighter jets, Chief Cabinet Secretary Osamu Fujimura said.

The Ministry of Defense said the incident involving a Chinese Oceanic Administration aircraft marked the first time a Chinese plane violated Japanese airspace, and threatened to escalate already tense Sino-Japanese relations, just days before a crucial Lower House election, where the conservative Liberal Democratic Party is expected to take back power.

The fixed-wing plane, marked "CMS" was spotted near Uotsuri Island in the East China Sea at 11:06 a.m. local time, prompting Japan's Air Self Defense Force to dispatch eight F-15 jets, according to the Ministry of Defense. Patrol boats in waters below warned the pilot to "stay out of our air space" over the radio, but the crew responded "this is Chinese air space," according to the Japanese Coast Guard.

The aircraft left without incident, but Fujimura called the move "extremely deplorable," on a day when Chinese ships were also spotted in territorial waters.

"It is extremely regrettable that, on top of that, an intrusion into our airspace has been committed in this way," he said.

Sino-Japanese relations have reached new lows since September, when the Japanese government purchased disputed islands - known as Senkaku in Japan, and Diaoyu in China - from private owners. The move triggered anti-Japanese protests and a trade dispute between Asia's two largest economies.

Chinese and Japanese boats have been involved in a near daily cat-and-mouse chase around the islands, but analysts say the latest incident threatens to escalate the spat to a whole new level.

"Military assets are now being used. It's a major escalation of the problem," said Narushige Michishita, Associate Professor at the National Graduate Institute for Policy Studies. "[China] is further undermining its relationship with Japan."

China's State Oceanic Administration posted aerial images taken from the plane in question on their website, and said its aircraft conducted joint patrols with four surveillance ships. A statement released by the agency, said the fleet declared the Chinese government's stance and ordered Japanese ships to leave China's territorial waters.

Michishita said today's incident could strengthen the opposition LDP's cause, days before voters head to the polls in an election that is expected to shift Japanese policy to the right. Party leader and former Prime Minister Shinzo Abe has called for a revision of Japan's pacifist constitution and a stronger military, to counter a rising China.

"This may help conservative politicians who are not very eager to improve Japan's relations with China," he said. "That wouldn't benefit either country."