(Adds Rouhani quotes)

By Parisa Hafezi

ANKARA, March 26 (Reuters) - Iran demanded an immediate halt to Saudi-led military operations in Yemen on Thursday and said it would make all necessary efforts to control the crisis there, Iranian news agencies reported.

Warplanes from Saudi Arabia and Arab allies on Thursday struck Iran-allied Houthi forces fighting to oust the country's Western-backed president. Saudi-owned al-Arabiya TV reported a ground offensive was being prepared.

"The Saudi-led air strikes should stop immediately," the Students News Agency ISNA quoted Iranian Foreign Minister Mohammad Javad Zarif as saying.

"We will make all efforts to control the crisis in Yemen," Zarif said, according to the agency's report from the Swiss city of Lausanne, where he is negotiating with six world powers to resolve a years-old dispute over Tehran's nuclear ambitions.

In a phone call with British Prime Minister David Cameron on Thursday, Iranian President Hassan Rouhani criticised the air strikes in Yemen, the semi-official Fars news agency reported.

"The regional countries should avoid any measures that can intensify the crisis," Rouhani said.

Violence has spread across Yemen since last year, with Houthi militia seizing the capital Sanaa and sidelining President Abd-Rabbu Mansour Hadi. It has made Yemen a front in Saudi Arabia's region-wide rivalry with Shi'ite-dominated Iran.

Tehran denies providing money and training to the Shi'ite Houthi militia, as alleged by some Western and Yemeni officials.

Yemeni Foreign Minister Riyadh Yaseen called on Monday for Gulf Arab help to prevent the Houthis from taking control of Yemeni airspace.

The Foreign Ministry in Tehran said the Saudi-led attack violated international norms and was dangerous. "It will lead to the spread of terrorism and extremism in the Middle East," Fars quoted Foreign Ministry spokeswoman Marzieh Afkham as saying.

Negotiations between Iran and six powers in Lausanne are aimed at striking a detailed political understanding by the end of March and reach a full agreement by June 30.

Saudi Arabia fears that the atomic deal would leave the door open to Tehran gaining a nuclear weapon, or would ease political pressure on it, giving it more space to back Arab proxies opposed by Riyadh.

"I am concerned by the impact of regional and international events on the nuclear talks," Iran's nuclear chief Ali Akbar Salehi told reporters in Lausanne, saying there were people "who are trying to ensure there is no deal." (Additional reporting by John Irish in Lausanne, Writing by Parisa Hafezi; Editing by Tom Heneghan and Crispian Balmer)

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