UNITED NATIONS - UN Secretary-general Ban Ki-moon, who has just received a letter from Iran on kidnapped Iranians in Syria, opposes any act of hostage-taking, a UN spokesman told reporters here on Wednesday.

"A letter from the government of Iran requesting the assistance of the secretary-general has been received," said Martin Nesirky, spokesperson for Ban, at a daily briefing when asked about the Iranian citizens being held in Syria.

"As a matter of principle, the secretary-general condemns any taking of hostages, and calls for the humane treatment, prompt and unconditional release, and safe return of any abductees, Iranians or others, being held against their will," he said.

A total of 48 Iranian pilgrims were reportedly abducted on Saturday by militants in the Syrian capital Damascus.

According to the letter sent by Iran's Foreign Minister Ali- Akbar Salehi to the UN secretary-general on Tuesday and obtained by Xinhua, three of the Iranian captives have reportedly been killed.

"I would like to draw your attention to the media reports indicating the killing of three Iranian captives in Syria," Salehi said in the letter to Ban. "The three captives were among 48 Iranian pilgrims who were abducted by the fighters from the so- called 'Free Syrian Army' on their way to the Damascus Airport on August 4, 2012."

He cited the same reports as saying that the hostage takers have threatened to kill the remaining captives in the coming hours.

"I would like to seek the cooperation and the good offices of your excellency for securing the release of these hostages," Salehi added.