Japan has confirmed Chinese authorities have arrested a third Japanese citizen and detained another on suspicion of spying.

The series of arrests and detentions could strain already tense ties between Asia's two largest economies.

Tokyo's top government spokesman Yoshihide Suga said Friday that a Japanese woman, who had been detained in Shanghai in June, was formally arrested in November.

He also said a Japanese man has been under criminal detention after being held in Beijing in June.

In September, China said it had arrested two Japanese citizens for suspected spying.

A total of four Japanese are now being held by Chinese authorities on suspicion of espionage.

"Japan does not engage in such (spying) activities in any countries," Suga told reporters, without elaborating.

Japan and China have taken steps over the past year to improve ties but relations remain shaky.

Tokyo is particularly wary as Beijing becomes increasingly aggressive in pressing its various sovereignty claims, including a dispute with Japan over ownership of a group of islands.

'Armed' China ship near disputed isles: Japan
Tokyo (AFP) Dec 26, 2015 - A Chinese coast guard ship which appeared to be armed with several cannon Saturday entered what Tokyo regards as its territorial waters near disputed islands, the Japan Coast Guard said.

Japan and China routinely butt heads over ownership of the uninhabited East China Sea islets, as Chinese ships -- mostly coast guard vessels -- and aircraft have sometimes approached them to back up Beijing's claims and test Japan's response.

It was however the first time an apparently armed Chinese coast guard vessel had "entered the territorial waters near the Senkaku Islands", the Japan Coast Guard said.

Japan administers the uninhabited islands under that name but China also claims them and calls them the Diaoyus.

On Tuesday Japan said it had spotted the armed Chinese coast guard ship for the first time in the contiguous waters near the islands.

"The ship is seen carrying four pieces of equipment, two at the front and another set of two at the rear, which each seem to have something similar to a cannon," a coastguard spokeswoman had said at the time.

The same ship entered what Tokyo considers its territorial waters, with two other Chinese coast guard vessels without such equipment, at around 9:30 am (0030 GMT) Saturday and stayed for about an hour, the Japan Coast Guard said in statements.

In November, Japan said it spotted a Chinese naval intelligence ship operating near the disputed islands for the first time.

Relations between Japan and China hit a low after Tokyo in September 2012 moved to increase its formal control by nationalising some of the islands.

But the countries -- Asia's two biggest economies -- have taken steps over the past year to improve ties.

They issued carefully worded statements on the dispute ahead of a summit in November last year in Beijing between Chinese President Xi Jinping and Japan's Prime Minister Shinzo Abe.

The two sides acknowledged they had different views on tensions emanating from the issue but agreed on the need to keep them under control.

Distrust, however, remains high as China is wary of moves by Abe to raise Japan's military profile while Tokyo frets about Beijing's increasing regional and global assertiveness.