Taiwan has been striving to free two senior intelligence agents kidnapped by China and held for seven years, the island's spy chief confirmed Wednesday.

Chu Kung-hsun and Hsu Kuo-chang, both colonels in the Military Intelligence Bureau, were kidnapped in Vietnam and taken to China in 2006.

"The National Security Bureau and Military Intelligence Bureau have set up an ad hoc group tasked with rescuing the pair since I took up the present opening in May 2009," said National Security Bureau director Tsai Der-sheng.

"Through different channels, either in public or secretly, we've been working hard to seek their release for the past few years," he said in answer to questions in parliament.

Tsai, quoted by the state Central News Agency, declined to give details.

China still claims Taiwan as part of its territory awaiting reunification, by force if necessary, since the two split in 1949 at the end of a civil war.

Ties have improved markedly since Ma Ying-jeou of Taiwan's China-friendly government came to power in 2008.

But the two former arch-rivals still spy on each other.

In 2012 Taiwan's top court rejected an appeal by a former general jailed for life for spying for China, the toughest punishment meted out in an espionage case in decades.

Lo Hsien-che, the highest-ranking official ever to be convicted of spying in Taiwan, reportedly started working for China in 2004.

He was suspected of handing over information relating to a project that gave the Taiwanese military some access to US intelligence systems.

According to Taiwanese media reports, he fell for a honey trap set by a female Chinese agent while stationed in Thailand and received about $1 million from China for his services.