Taiwan's premier warned Tuesday of increasing cyber attacks from China, the island's highest-ranking official to denounce attempts to "steal government secrets" despite warming cross-strait relations.

Jiang Yi-huah's warning comes after the Federal Bureau of Investigation warned on Sunday that China is waging an aggressive cyber war against the United States which costs American businesses billions of dollars every year.

"The mainland's cyber units have been stepping up attacks on Taiwan's civil and government websites," Jiang said in parliament.

Urging government employees to heighten their vigilance, Jiang said Chinese cyber attackers often use civil websites as springboards to access Taiwan's official websites and "steal government secrets".

Taiwan and China split in 1949 at the end of a civil war, separated by the Taiwan Strait.

Ties between Taiwan and China have improved markedly since President Ma Ying-jeou took office in 2008 on a platform of boosting cross-strait trade and tourism.

But Taiwanese government websites have frequently suffered digital bombardments from China, usually during disputes between the two sides, authorities say.

"The Chinese mainland has political ambitious towards Taiwan and never ceased its preparations for invading Taiwan, among them military exercises and cyber attacks," Jiang said.

"While the cross-strait ties look calm ostensibly, the undercurrents have been turbulent," he added.

The United States, a key ally of Taiwan, routinely accuses China of cyber attacks and spying. Five Chinese military officers were indicted for hacking into US companies in May.