Taiwan slammed a unilateral move by China to open four new flight routes over the Taiwan Strait as "unacceptable" Tuesday, saying it risked allowing planes to fly too close to aircraft on existing routes.

China on Monday announced it would start flying four new routes from coastal Zhejiang province and the cities of Fuzhou and Xiamen in Fujian province over the Taiwan Strait.

While China had consulted with Taiwan over one of the new routes, M503 -- without reaching an agreement -- it had not discussed the other three flight routes W121, W122 and W123, due to start operations from March 5.

"They unilaterally announced the new routes before reaching an agreement with us," Taiwan's Civil Aeronautics Administration said in a statement.

"The measures are unacceptable to us. We demand the mainland side continue consulting with us", it said, adding that the move breached the Convention on International Civil Aviation.

Sources said the Chinese authorities hope to use the routes to ease congestion on existing paths caused by the growing volume of air traffic.

But Taiwan said the routes represented a potential air defence threat, with M503's proposed path very close to skies in the Strait administered by Taiwan.

"The military will monitor, intercept and disperse (unidentified aircraft and ships) according to the established measures so as to safeguard the security of territory", Taiwan's defence ministry said in a statement.

Ties between China and Taiwan have improved markedly since 2008 after Ma Ying-jeou from the China-friendly Kuomintang party came to power. He was re-elected in 2012.

However, many Taiwanese remain wary of Beijing's increasing influence over the island. A planned pact to free up the services trade with China sparked an occupation of Taiwan's parliament and mass street protests last year.

China considers self-ruled Taiwan a part of its territory awaiting reunification -- by force if necessary. They split in 1949 at the end of a civil war.