TAIPEI (AFP) - China on Sunday tested a proposed new flight route in the Taiwan Strait in a plan that had sparked a fierce backlash from the island over security concerns, officials said.

Beijing has insisted the new route and three others that had all been scheduled to open on March 5 are necessary to ease congestion on an existing flightpath, but Taiwan has slammed the unilateral move as "unacceptable" and said it poses a potential air defence threat.

Inaugurating the four routes, which would take planes over the Taiwan Strait from China's coastal province of Zhejiang and the cities of Fuzhou and Xiamen in Fujian province, was postponed after protests from Taiwan.

Taiwanese authorities said Sunday they were informed by their Chinese counterparts that mainland officials had begun testing the M503 flight route from 0300 GMT (11:00 am Singapore time) onwards.

The test involves flying near the middle of the Taiwan Strait.

"We have been informed that the mainland side began testing the M503 flight route from 11:00 am," Taiwan's Civil Aeronautics Administration (CAA) said in a statement.

Taiwan's defence ministry said it was keeping a close eye on the tests, which the CAA said were aimed at examining the accuracy and proficiency of the planned route regarding navigation, radar and communication issues.

China has discussed issues related to the M503 route with Taiwan, but no agreement has so far been reached, Taiwanese authorities said.

They added that Beijing had not mentioned the three other proposed routes.

Just days ahead of the scheduled March 5 opening of the four flights, Taiwan said China had agreed to postpone the inauguration of the planned routes and the M503 route will be moved slightly closer to the mainland in what some local media said was a conciliatory move.

Cross-strait ties have improved markedly since Ma Ying-jeou from the China-friendly Kuomintang party came to power in Taipei in 2008. 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.