By Dominic Evans

RIYADH, Nov 12 (Reuters) - Saudi Arabia, rocked by a deadly suicide attack, ruled out any dialogue with violent Islamists and said it would only talk to them with the rifle and sword.

The uncompromising message by Interior Minister Prince Nayef came after four Muslim clerics offered to mediate with militants to avoid further bloodshed like Sunday's suicide attack by suspected al Qaeda militants in Riyadh which killed 18 people.

"We will not hold any dialogue with them other than with the rifle and the sword," Prince Nayef said during a visit to Tunisia. His remark was carried by Saudi media on Wednesday.

Mohsen al Awajy, a moderate Islamist who was jailed in the 1990s for opposing the Saudi government, said the clerics were working hard on an initiative to persuade militants to give up violence and had got an "encouraging response from officials".

Awajy said the clerics made their mediation offer last week, after a succession of security raids on militants in Riyadh and Mecca killed five people but before the bombing.

His comments appeared at odds with a pledge from King Fahd to strike with an iron fist against the suspected al Qaeda militants behind the attack. Fahd's tough message was then reinforced by his security minister.

Saudi-born Osama bin Laden's al Qaeda network is suspected of staging Sunday's attack in which at least 18 people died, including five children, and 120 were wounded in Riyadh.

Most of the victims were Arabs. In May, 35 people died in a triple suicide bombing on a housing complex in the capital, including nine Americans.

Saudi officials and Washington blame al Qaeda for the weekend attack in which bombers posing as police blew up an explosives-rigged car in the Muhaya compound.

The mediation offer followed a similar move after the May suicide bombings.

One of the clerics involved in both initiatives, Sheikh Safar al Hawali, was reported as saying he helped secure the surrender of a leading militant to Saudi authorities.

That report was later denied by Prince Nayef, but officials have often said that militants who give themselves up voluntarily can expect a more sympathetic court hearing.

Sources close to the investigation have said a number of people were detained for questioning but Nayef said no one involved in the latest attack had been arrested.

Washington has been pressing Saudi Arabia to combat al Qaeda, which it holds responsible for the attacks on U.S. cities on September 11, 2001. Fifteen of the 19 attackers were Saudis.

U.S. Deputy Secretary of State Richard Armitage, speaking in Riyadh earlier this week, said al Qaeda was now determined to overthrow the House of Saud.

"It's quite clear to me that al Qaeda wants to take down the royal family and the government of Saudi Arabia," he said.