US Defense Secretary Robert Gates appealed to Saudi leaders on Wednesday to back a US-led drive for tough sanctions against Iran in a visit to Riyadh and came away "hopeful," his press secretary said.

Gates met King Abdullah and the crown prince as part of the Obama administration's diplomatic push for fresh UN sanctions on Iran, seeking help from an oil-rich ally amid continued opposition from China.

"We are certainly hopeful that the Saudis will use whatever influence they have which is considerable in this region and throughout the world to try and help us in our efforts at the UN so we can get meaningful sanctions enacted against Iran," press secretary Geoff Morrell told reporters.

After having attempted to engage Iran in a conciliatory dialogue, the Obama administration has vowed to pile pressure on the Islamic republic to persuade it to abandon its uranium enrichment work.

Saudi leaders were generally "supportive" of the approach, said a US defense official, who attended the talks and spoke on condition of anonymity.

The Saudis, who have yet to openly endorse more sanctions, might choose to exercise their diplomatic influence discreetly, the official told reporters.

Washington has turned to the Saudis to persuade Beijing not to block sanctions against Tehran by guaranteeing China stable oil supplies in the event of disruption from Iran.

Saudi Arabia and other Arab states in the Gulf are "incredibly concerned about Iran's nuclear programme," as well as its growing missile arsenal and "destabilising" role in the region, the official said earlier.

The diplomatic climate has changed since Gates last visited Riyadh in May, when he had to reassure an anxious Saudi leadership that President Barack Obama's offer of dialogue with Tehran would not jeopardise Washington's close ties with the kingdom.

The UN Security Council has already imposed three rounds of sanctions on Iran over its refusal to halt uranium enrichment which Israel and the West view as a cover to build nuclear weapons.

Tehran denies the charge, saying the programme is for peaceful nuclear energy.

Gates held talks with the king at a desert palace on the outskirts of Riyadh, including a 45-minute one-on-one with the monarch.

He earlier met Crown Prince Sultan bin Abdul Aziz, the king's half-brother and minister of defence and aviation since 1962.

The Pentagon chief also discussed bolstering Saudi "air and missile defence capabilities" as part of a broader US effort to boost security in the Gulf in the face of Iran's expanding arsenal of ballistic missiles, the official said.

The United States has promised to speed up weapons sales to Saudi Arabia and other Gulf allies, which have bought billions of dollars worth of American weapons in recent years.

US officials believe the arms buildup in the Gulf sends a clear signal to Iran that its nuclear and missile programmes are counter-productive.

"It's not lost on the Iranians all the security cooperation that's been going on for years now," Pentagon press secretary Geoff Morrell said before the talks.

The warplanes and missile defence systems bought by the Saudis, the United Arab Emirates and other states were "all designed to counter-weight and protect against the growing threat posed by Iran," Morrell said.

"As Iran becomes more menacing, we're seeing greater (security) cooperation" among Gulf states that previously had been reluctant to work together, he said.

Apart from Iran, Gates also discussed efforts to fight Al-Qaeda's branch in neighbouring Yemen, blamed for attempting to blow up a US-bound airliner on Christmas Day, officials said.

Days after Iraq's elections, Gates planned to renew US appeals for Saudi to step up its diplomatic engagement with Baghdad, despite the kingdom's suspicions about Iranian influence with Iraq's leading Shiite political parties.