An Arab alliance attacking Shi'ite Muslim Houthi forces in Yemen initially plans a month-long campaign, but the operation could last five or six months, a Gulf diplomatic official said on Saturday.

The official, from a country that belongs to the alliance, said Shi'ite Iran, the Houthis' main foreign ally, was likely to retaliate indirectly, by encouraging pro-Iranian Shi'ite activists to carry out armed attacks in Bahrain, Lebanon and eastern Saudi Arabia.

Iran, in a tug-of-war with Sunni-ruled Saudi Arabia for influence across the Middle East, has denied supporting the Houthis militarily, and has criticised the Gulf Arabs for their military action.

Since the Saudi-led coalition launched air strikes against them early on Thursday, the Houthis, seeking to overthrow the Western- and Saudi-allied President Abd-Rabbu Mansour Hadi, have continued to make gains.

But the official, who declined to be identified due to the sensitivity of the matter, said the attacks would go on until Yemen was able to resume a U.N.-backed political transition interrupted by the Houthis' seizure of Sanaa in September.

He said Gulf Arab concern over the Houthis' influence in Yemen had been heightened in January by satellite imagery showing Houthi forces repositioning long-range Scud missiles in northern districts near the Saudi border.

The Scuds, with a range of between 250 km (150 miles) and 650 km (400 miles), were aimed northwards at Saudi territory.

He said Yemen's military had about 300 Scuds, the bulk of which were believed to be in the hands of the Houthis and allied military units loyal to former president Ali Abdullah Saleh, and that the campaign so far had destroyed 21 of them.

Struggle for stability

Yemen, by far the poorest country on the Arabian peninsula, has struggled to regain stability since mass protests in 2011 that eventually unseated Saleh after 33 years in power. Hadi led a national dialogue that was discussing a new constitution when the Houthis took the capital and pushed him aside.

The Gulf diplomatic official said the coalition would not accept that the Houthi "coup" had succeeded, and wanted Yemenis to push for a resumption of the U.N.-backed process.

He said it could take five or six months for the campaign's aims to be realised, but there was room for everyone, including Houthis, in that process of forging a new constitution.

The official said Houthi forces were being trained and supported on the ground by about 5,000 experts from Iran and its regional allies, the Hezbollah group in Lebanon and Iraqi Shi'ite militias.

He said the Houthis were today a "quintessentially Yemeni tribal" movement, but with a few years of Iranian training would become a more formidable force, capable of ruling much of the country.

It was widely recognised that Hadi lacked a significant power base, the official said.

But Arab states supported Hadi as he was Yemen's legitimate head of state and had a role as a temporary, transitional figure leading the U.N.-backed reform process, which was meant to shepherd Yemen to stability after decades of autocracy followed by political upheaval.

Saleh's political party called on Friday for a cessation of hostilities by both sides, a statement carried by the party's website said. The Gulf diplomatic official welcomed that, but said it would be good to hear Saleh say it in his own words.

