BEIJING (REUTERS) - China said on Wednesday Taiwan should not be so suspicious of its intentions after Taipei protested against Beijing's decision to open new domestic commercial air routes over the Taiwan Strait separating the diplomatic rivals.

The Taiwan Affairs Office of China's State Council said Taiwan must be "more understanding and less suspicious" of the four routes, China's state media said.

Taiwan said on Tuesday it would step up air surveillance patrols in the Taiwan Strait in response "if necessary".

China deems Taiwan a renegade province and has not ruled out the use of force to take it back, particularly if the island makes a move towards independence.

Beijing's "one China" policy holds that there is only one China and that Taiwan is part of it.

Taiwan's Ministry of Transportation and Communication said in a statement on Tuesday that Beijing's decision to open the new routes, which go into effect on March 5, was "unilateral"and "not acceptable" and could increase flight-safety risks, particularly around Taiwan's outlying islands of Kinmen and Matsu.

The routes are between eastern Shanghai and the southern Pearl River Delta.

The two sides had been discussing a new north-south air route on the west side of the median line dividing the Taiwan Strait, but talks had not yet finished, the ministry said.

The United States, which supports a "one China" policy but provides security assistance to Taiwan, said its primary focus on the air-routes issue was "maintaining and enhancing international aviation safety".

"We do encourage China to engage and consult with the parties affected by the newly declared air routes in the Taiwan Strait to ensure that air safety concerns are addressed," State Department spokeswoman Marie Harf told a regular news briefing.

While Taiwan and China have signed a series of landmark trade and economic agreements since 2008, political and military suspicions still run deep, especially in democratic Taiwan where many fear China's true intentions.