US Secretary of State Hillary Clinton held long talks with Saudi leaders on Monday aiming to rally support for tough new UN sanctions against Iran, which she warned is turning into a "military dictatorship" bent on building a nuclear bomb.

Clinton and King Abdullah met for talks and a sumptuous lunch with tables overflowing with food, at Rawdat Khurayim, the monarch's desert camp 60 miles (35 miles) northeast of Riyadh, in a tent-topped air conditioned building.

No details immediately emerged as subsequent private talks ran over two-and-a-half hours.

She flew in from Qatar earlier on Monday on her first visit to the oil-rich kingdom, after using some of the strongest language yet about events in Iran from an administration which just a year ago had sought to hold out the hand of friendship.

The chief US diplomat first went straight into talks with her Saudi counterpart Prince Saud al-Faisal at Riyadh airport before heading to the desert camp.

Aides said she would press Saudi leaders to use their influence with China to secure a change of heart on sanctions against Iran over its controversial nuclear programme.

China appears to be the strongest holdout to sanctions among the five veto-wielding permanent members of the UN Security Council.

Clinton's top assistant for the Middle East, Jeffrey Feltman, told reporters travelling with her that China had an "important trading relationship" with the Saudi oil kingpin.

"We would expect them (the Saudis) ... to use their relationship in ways that can help increase the pressure that Iran feels," said Feltman, the assistant secretary of state for Near East Affairs.

Speaking to students earlier in neighbouring Qatar, just across the Gulf from Iran, Clinton said the whole region had reason to fear Iran's nuclear programme and the growing influence of the elite Revolutionary Guards.

Clinton said the United States was not aiming to use military action to curb Iran's nuclear ambitions but rather seeking to build support for tough new sanctions at the UN Security Council.

She said the package Washington wanted adopted "will be particularly aimed at those enterprises controlled by the Iranian Revolutionary Guard, which we believe is in effect supplanting the government of Iran.

"We see the government of Iran, the supreme leader, the president, the parliament is being supplanted and Iran is moving toward a military dictatorship," she said.

"They are in charge of the nuclear programme.

The Revolutionary Guards continue to be the military guardian of Iranian supreme leader Ayatollah Ruhollah Khomeini's Islamic ideology, but also own large tranches of Iran's economy.

The United States last week imposed a fresh round of sanctions against the elite force and hopes for UN sanctions.

In Washington, Fariborz Ghadar, senior adviser at the Centre for Strategic and International Studies, said Clinton appeared not to be up to speed with developments in Iran.

"We should have known about this trend when two-thirds of the 21-man first cabinet of Ahmadinejad were IRGC (Islamic Revolutionary Guard Corps) colleagues," Ghadar told AFP in an email.

"Then most major contracts were given to the IRGC and now the regime relies on the IRGC's brute force for their survival," he said.

"When you rely on the power of the IRGC to remain in power, it is only a matter of time before the regime becomes a paramilitary dictatorship -- it is about time we realise this," he said.

In Riyadh, Saudi leaders were also expected to raise the Middle East peace process in their talks with Clinton amid growing frustration with the failure of US efforts to secure a relaunch of talks frozen since Israel launched its devastating offensive against Gaza in December 2008.

"The peace process is the main issue, of course," said Saudi foreign ministry spokesman Osama Nugali. "Our position is still the same... that we need to revive the peace process."

In Qatar, Clinton said she was optimistic that talks would resume this year.

"I'm hopeful that this year will see the commencement of serious negotiations," she said.

US Middle East peace envoy George Mitchell has proposed that Israel and the Palestinians hold indirect talks over a three-month period to get round Israel's refusal to accept Palestinian demands for a complete freeze on settlement construction before any direct negotiations.

But the idea has met with little enthusiasm from the Palestinians or their regional backers, including Saudi Arabia.

"They could be labelled the proximity talks but the more apt description is the nonsense of non-talks," the government-linked Saudi daily Arab News commented on Monday.