Saudi-led airstrikes alone are unlikely to crush Iranian-backed Yemeni rebels but a ground incursion would risk a bloody "quagmire" and escalating tensions with Tehran, experts say.
Saudi Arabia pledged to do "whatever it takes" to defend its ally President Abedrabbo Mansour Hadi after launching aerial raids against the Huthi Shiite fighters and their allies.
Despite the kingdom's formidable firepower, analysts say dropping bombs alone is of limited effectiveness.
"History shows that airstrikes without corresponding ground forces do not produce a decisive victory," said Frederic Wehrey of the US-based Carnegie Endowment for International Peace.
He said this week's raids appeared to be aimed at fixed, pre-identified targets like airbases and command centres, rather than mobile Huthi units, urban fighters or supply lines.
The coalition said that anti-aircraft defence systems, missiles and artillery positions "were completely destroyed" on the first day of strikes.
But the raids risk provoking a backlash among the civilian population if the death toll rises.
Officials at the rebel-controlled health ministry said Friday that dozens of civilians had been killed in more than 24 hours of Saudi-led raids.
"If Saudi Arabia relies only on airstrikes and civilian casualties start mounting they will lose support very, very quickly," said Sultan Barakat, research director at the Doha-based Brookings Institution.
- 'No clear exit' -
Sending in ground troops is seen as a last resort given the risks.
"It is hard for me to see the Saudis deploying ground forces to eject the Huthis, given the likelihood of a quagmire without a clear exit," said Wehrey.
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A possible exception would be a buffer zone on the Saudi-Yemeni border, he added.
A Saudi spokesman for the Arab coalition carrying out operation "Decisive Storm" told reporters on the first day of strikes Thursday that there were no immediate plans to put boots on the ground.
A protracted conflict in Yemen could also empower Sunni extremists in the impoverished and deeply tribal country.
"Just crushing the Huthis will change the factional dynamics, which could help ultra-radical Sunni groups," said Jon Marks, a Middle East expert at London-based think-tank Chatham House.
Yemen is home to what the United States considers the deadliest branch of Al-Qaeda, which claimed the Islamist attacks in Paris in January and has repeatedly clashed with the Huthis.
The rival Islamic State jihadist group has also surfaced in Yemen, claiming responsibility for attacks on Huthi mosque that killed 171 people in one week.
The Huthis, whose influence had long been confined to the mountainous north, belong to the Zaidi sect of Shiite Islam -- a minority in Yemen where almost 70 percent of the population is Sunni.
In their push towards central and southern regions, the Huthis received support from powerful military units loyal to former strongman Ali Abdullah Saleh, himself a Zaidi who was forced to resign in 2012 following a year-long uprising.
The wealthy ex-president has remained influential, enjoying the loyalty of commanders of many army units that he built during his three-decade rule.
Yemen has long been the scene of clashes between rival factions and tribes, and experts agree that the Saudi-led intervention is unlikely to restore stability.
"The prospect of Saudi Arabia or other Arab states putting regular troops on ground in Yemen will remain remote," said Britain-based consultancy group Verisk Maplecroft.
Saudi Arabia "will not want to get bogged down in a conflict which could morph into a protracted counter-insurgency -- and risk a more direct confrontation with Iran," it said.
The White House has voiced concern about "reports of Iranian flow of arms into Yemen".
And Saudi Arabia's ambassador to the United States, Adel al-Jubeir, accused Iran of "aggression" across the region and backing the Huthi power grab.
Saudi Arabia has reportedly mobilised 150,000 troops near the border but said Thursday it has no plans yet for a ground operation.
A ground assault would be a "significant climb up the escalatory ladder toward Iran," Wehrey said.
The Saudis "may use the air operation to have greater leverage in negotiations for some sort of power-sharing agreement," he added.