Vienna - The daughters of Chinese biochemist and businessman Wo Weihan, who was executed in China last week on espionage charges, said Monday that their father had been forced to make his confession.

Ran Chen and Di Chen, both Austrian citizens, made the claim in an open letter to the Austrian government, which had been fighting to prevent Wo's execution.

"The verdict was made under unclear circumstances, but certainly on the basis of a confession made under violent duress, which was revoked by our father," the women said in the letter.

Wo, 60, was executed last Friday after having been convicted in 2007 of passing secret Chinese military data to Taiwan.

As Wo lived in Austria in the 1990s, the country's government sharply protested the execution and called in the Chinese embassy's charge d'affaires to the Austrian foreign ministry on Friday.

The European Union presidency, currently held by France, also condemned China's action.