SANAA / ADEN / RIYADH: A coalition of regional countries led by Saudi Arabia laid siege to conflict-ridden Yemen Thursday, launching airstrikes against Houthi rebels and vowing to cut off any outside support for the Iranian-backed movement.

A coalition spokesman called the first wave of Saudi-led air raids “successful,” vowing to press on with the intervention until goals are reached.

Speaking to reporters in Riyadh, spokesman Ahmad Assiri also said that there were no immediate plans for a ground invasion but added that “if there is a need for ground forces, Saudi Arabia and friendly states are ready and will respond to any aggression of any sort.”

The spokesman vowed that the coalition would not allow any supplies to reach the rebels and that no party would be allowed to back the Houthi rebellion.

Early in the day, Saudi forces “dealt with armed terrorist groups heading toward Saudi Arabia’s southern borders,” Assiri said, without elaborating

He added that “all forms of aircraft” have been taking part in the strikes and that all of them had “returned safely to their bases.”

The operation would continue “as long as needed” until “legitimacy” represented by President Abed Rabbou Mansour Hadi and his government was restored in Yemen, he said.

Anti-aircraft defense systems, missiles and artillery positions “were completely destroyed” and the Saudi air force “completely” took control of Yemen’s airspace “within the first 15 minutes” of raids, Assiri said.

Egyptian military and security officials told the Associated Press that the military intervention would go further, with a ground assault by Egyptian, Saudi and other forces, planned once airstrikes have weakened the rebels.

In the air assault – codenamed “Operation Decisive Storm” – Saudi Arabia deployed some 100 fighter jets, 150,000 soldiers and other navy units, Al-Arabiya TV reported. The UAE, Bahrain, Qatar, Kuwait, Jordan, Morocco, Sudan and Egypt also took part, though it was not clear which carried out actual strikes.

The strikes that began before dawn barraged Sanaa and three other provinces, hitting a Sanaa air base, military bases and anti-aircraft positions – and flattened a number of homes near the capital’s airport, killing at least 18 civilians.

In a statement as the airstrikes began, Saudi Arabia, Kuwait, the UAE, Qatar and Bahrain said their action aimed to “protect” Yemenis from Houthis who are “a tool in the hands of foreign powers.”

In an angry, televised speech, rebel leader Abdul-Malik al-Houthi accused the United States, Saudi Arabia and Israel of launching a “criminal, unjust, brutal and sinful” campaign aimed at invading and occupying Yemen.

“Yemenis won’t accept such humiliation” he said, calling the Saudis “evil.”

Washington expressed concern about Iran’s alleged role in stoking the violence, as a National Security Council official told AFP Tehran’s actions were destabilizing and threatening Yemen’s government. “We have concerns about Iranian activities in Yemen and reports of Iranian flow of arms into Yemen,” Alistair Baskey said, adding that the moves were “contributing to destabilizing the situation and contributing to the threat posed to the legitimate government.”

The U.S. also announced it would provide “logistical and intelligence support” to the coalition, which officials said could cover refueling and early-warning radar aircraft support.

Iran called the airstrikes a “dangerous step,” Foreign Ministry spokeswoman Marzieh Afkham said.

“This invasion will bear no result but expansion of terrorism and extremism throughout the whole region,” she said.

Powerful explosions rocked Sanaa Thursday night and anti-aircraft fire erupted in response to what witnesses said were airstrikes by the coalition forces against a camp at Al-Istiqbal, at Sanaa’s western entrance.

Witnesses also said strikes targeted the Samaa military base, north of Sanaa, which is used by army units believed to take orders from their former commander, Ahmad Ali Saleh.

Saleh is the son of ousted President Ali Abdullah Saleh, accused of allying with the rebels.

Separately, Arab foreign ministers meeting in Egypt endorsed a draft resolution to form a unified military force, to counter growing regional security threats.

“The Arab ... ministers agreed on adopting an important principle, which is forming the unified Arab military force,” Arab League Secretary-General Nabil Elaraby told reporters in Sharm el-Sheikh.

“The task of the force will be rapid military intervention to deal with security threats to Arab nations,” Elaraby added.

The draft resolution will be on the agenda of this weekend’s Arab summit.

Over the past months, the Houthis have swept out of their northern strongholds to take over the capital and much of the north. The U.S.- and Gulf-backed president Hadi was forced to flee to the southern port of Aden, relying on the support of some police and military units and allied militiamen. But as the Houthis and their allies bore down on Aden, Hadi left the country by boat on Wednesday afternoon, security officials said.

Hadi reappeared Thursday evening, arriving by plane in Riyadh, Saudi state TV reported. A Yemeni security official told the AP the president had gone by boat to the Yemeni port of al-Mukalla in the western province of Hadramawt. The next day, he drove across the border into Oman and from there was flown to Riyadh.

The airstrikes appeared to boost the morale of troops and militiamen loyal to Hadi. In Aden, pro-Hadi militiamen battled in two districts with Houthi fighters backed by forces loyal to Saleh. Bodies of slain fighters were seen in the streets, as shops closed and residents took shelter inside their houses, witnesses said.