Prime Minister Dr Mahathir Mohamad today accused Singapore of using "unethical" tactics in a row over water that has sparked a tit-for-tat publishing duel.

Mahathir was angered by Singapore's recent publication of a booklet on the squabble, which contained copies of letters exchanged by national leaders over the controversial issue.

"That is unethical," Mahathir told reporters. "Now it's very difficult to write letters because we know it's going to be used for propaganda. It will be used as evidence against you."