Japanese fighter planes scrambled to the Senkakus after a lone Chinese state aircraft flew over the disputed islands on Thursday, a move seen as part of Beijing’s intensified efforts to challenge Tokyo’s control over the territory.

“It is extremely deplorable,” Japan’s chief government spokesman Osamu Fujimura told the Financial Times.

The fighter jets took off from nearby Okinawa. China has long accused Japan of stealing the islands, which it calls the Diaoyu, in the late 19th century and has stepped up its challenges to Tokyo’s control of the waters surrounding the islands in the East China Sea.

The incident was the first violation of Japan’s territorial airspace by a Chinese official aircraft since 1958 and would likely stoke tensions in the long-standing diplomatic row between the two countries, the Defence Ministry said.

Tensions have mounted since Japan purchased three of the islands, provoking massive protests across Beijing which further damaged bilateral relations.

China’s foreign ministry on Thursday said the aerial surveillance of the Diaoyu was “completely normal” and demanded Japan stop entering the islands’ waters and airspace.

The territorial dispute has become one of the top foreign policy campaign issues and could largely influence the ballot ahead of Japan’s general elections on Sunday.