By Frank Gardner BBC security correspondent

A senior Gulf Arab official has given British journalists the most detailed explanation yet as to why the Saudi-led coalition has gone to war against the Houthi rebels in Yemen.

With the conflict there now in its second month the UN estimates more than 1,200 people have been killed and thousands more displaced, amid a growing humanitarian crisis.

But according to the official, who asked not to be named, Gulf Arab governments decided they had to intervene to stop the Houthi takeover of Yemen after new intelligence emerged in January this year.

He said the US government had confirmed that satellite surveillance revealed some of Yemen's estimated 300 Scud missiles, under rebel control, had been moved close to the Saudi border.

With a range well in excess of 300km (186 miles), that would have placed several Saudi cities within striking range of the missiles, he said.

Saudi Arabia and its Gulf Arab allies were not going to tolerate what they call "an Iranian-backed militia taking over a country in their backyard", he said, adding that "the Gulf Arab states want an end to Iranian interference in Arab affairs".

They are determined to confront what they see as Tehran's expansionist ambitions across the Middle East.

When asked what proof he had of any material backing by Iran for Yemen's Houthi rebels, the official declined to give a specific answer, nor did he provide any photographic evidence of the alleged Scud bases on the Saudi border.

Iran denies giving any military support to the rebels.

But the Gulf official told invited journalists from several leading media organisations that Tehran had given extensive political support to the Houthis, who share Iran's Shia branch of Islam.

He said the assessment of the Arab Gulf Cooperation Council (GCC) was that before the Saudi-led coalition began its air campaign to dislodge the rebels on 26 March, there had been up to 5,000 Iranian and Iraqi trainers in Yemen, helping the rebels.

At least three trainers from Iran's Revolutionary Guards Corps (IRGC) had been caught in Aden, he said, and handed over to Oman. It is the only Gulf Arab state to have maintained a neutral stance in the Yemen war.

Growing combat experience

Among the hundreds of Gulf Arab military pilots and crews deployed at Saudi airbases for the air war over Yemen, a growing number now have significant combat experience, said the official.

This has been acquired in flying missions over Syria, Iraq, Libya and Kosovo as well as Yemen.

The Saudis are flying the most sorties, followed by the Emirati pilots. Gulf Arab states are among the most prolific buyers of western military equipment in the world, spending large amounts of oil revenues on sophisticated hi-tech aircraft and munitions.

Yemen, by contrast, has an ageing arsenal of mostly Soviet-era weapons with a huge number of AK47 machine-guns in private hands.

Saudi Arabia announced an end to the initial air campaign over Yemen on 21 April after coming under US pressure to halt the bombing.

Washington was fearful of derailing its proposed deal with Iran to place curbs on its nuclear programme.

But the official said the very next day Houthi rebels took six of their MiG fighter planes out from cover and began a renewed onslaught on government defenders in Taiz and Aden.

This, said the official, was why coalition air strikes had resumed.

The Saudi-led coalition now has control of Yemen's air space and most of its ports, putting in place an arms embargo on the whole country.

Because of the worsening humanitarian situation in Yemen, the Saudis were allowing in aid flights but insisted planes from Iran needed to stop off first at a Saudi base in Bisha to be searched for arms.

The official said that an Iranian plane was detected flying direct to the Yemeni capital Sanaa on 28 April and that its pilots had ignored calls from Saudi and Omani air traffic control.

Suspecting it of carrying weapons, the coalition then responded by bombing the runway at Sanaa to prevent it landing, while the Yemeni government-in-exile complained to the UN Security Council that Iran was trying to break the arms embargo.

'Yemen's Machiavelli'

The Gulf official admitted that early attempts to broker a peace deal with Yemen's deposed former president Ali Abdullah Saleh through his son had failed.

Saleh, who the official dubbed "Yemen's Machiavelli" for his constant behind-the-scenes scheming, ruled the country for more than 20 years before he was driven out by the Arab Spring protests in 2012.

But the officials said that within the military, 36 brigades had been run by his son and a further 17 by his son-in-law.

Yemen's officer corps was largely drawn from the same Zaidi Shia sect that the Houthi rebels come from. The Zaidis make up around 30% of Yemen's population.

Much of the army is still loyal to Saleh, who is still "manoeuvring" but, he said, the Gulf Arabs are hopeful they can eventually break the current alliance between ex-President Saleh and the Houthi rebels, which had "opened the door to Iran in Yemen".

Gulf Arab leaders have a number of expectations from the upcoming summit with US President Barack Obama at Camp David on 14 May, the official said.

Above all, they want US reassurance that their own security concerns about Iran are not going to be ignored in the White House's push for a nuclear deal with Tehran.

If the summit ends with little more than a photo-op and a statement then it would be a disappointment, he said.

Instead, the GCC are hopeful of a clear agreement, perhaps a formal memorandum, that settles on measures to "contain Iranian influence".

He added that the US administration still needs to "sell" the proposed Iran agreement to its Gulf Arab allies.

Iran, he maintained, is very adept at building client militias in the region and has ambitions in six countries, naming them as Lebanon, Syria, Iraq, Bahrain, Yemen and eastern Saudi Arabia.

Buying tribal allegiances

The Saudi-led coalition has come in for considerable criticism, notably from human rights groups, for its extended bombing campaign which has seen mounting civilian casualties.

But the Gulf official said there is a multi-pronged strategy, which comprises maintaining control of the air and sea, enabling humanitarian aid, planning reconstruction and development and buying the allegiance of tribal leaders.

"This has already begun, but it will take time," he said.

Yemen needs a political solution - that, at least, everyone is agreed on.

But the Gulf Arabs want a return to the political road map, sanctioned by the UN Security Council, while Saleh and the Houthis want to renegotiate on their terms.

The Gulf Arab official concluded his briefing by telling journalists that Arab governments in the Saudi-led coalition will do "everything that is required to win in Yemen".

He said the days of maintaining a policy of what he called "benign negligence" in that country are over.

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