Saudi Arabia bombed key military installations in Yemen on Thursday, leading a regional coalition in a campaign against Shiite rebels who have taken over much of the country and drove out the president. The dramatic military assault turns impoverished, fragmented Yemen into a new front in the conflict between Saudi Arabia and Iran.

Egyptian military and security officials told The Associated Press that the military intervention will go further, with a ground assault into Yemen by Egyptian, Saudi and other forces, planned once airstrikes have weakened the capabilities of the rebels, known as Houthis, and their allies, military forces loyal to former president Ali Abdullah Saleh.

The strikes before dawn barraged an air base near the airport in the capital, Sanaa, as well as anti-aircraft positions and military bases -- and flattened a number of homes near the airport, killing at least 18 civilians. The Houthis mobilized thousands of supporters in protests against the strikes, with one speaker lashing out at the Saudi-led coalition and warning that Yemen "will be the tomb" of the aggressors.

U.S. Secretary of State John Kerry expressed support for the strikes in a conference call with Gulf foreign ministers and said the U.S. shared intelligence against Houthi targets, according to a State Department official traveling with Kerry in Lausanne, Switzerland.

Iran, which is allied to the Houthis, denounced the bombing, noting the civilian deaths. Iran "considers this action a dangerous step," Foreign Ministry spokeswoman Marzieh Afkham said in a statement. "This invasion will bear no result but expansion of terrorism and extremism throughout the whole region."

More On This...

The sudden internationalization of the Yemen conflict brewing for months throws a new convolution in the twisted threads of conflict in the Middle East. The new tension puts the United States, a traditional ally of Saudi Arabia, in a precarious situation with Tehran as it tries to negotiate a nuclear deal before the end of this month.

In Iraq, the U.S. and Iran are implicitly on the same side -- both helping the Shiite-led government in Baghdad battle Islamic State group militants, though Tehran and Washington are intently avoiding any actual contacts. In Yemen, the U.S. is backing the Gulf and its allies against Shiite rebels allied to Iran -- while at the same time, Al Qaeda's branch in the country is also fighting the Shiite rebels.

Saudi Arabia and fellow Sunni-led allies in the Gulf and the Middle East view the Houthi takeover as a move by Iran to establish a proxy on the kingdom's southern border. Iran and the Houthis deny that Tehran arms the rebel movement, though Tehran says it gives it diplomatic and humanitarian support.

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

Over the past months, the Houthis have swept out of their northern strongholds to take over the capital and much of the north. Hadi, the U.S.- and Gulf-backed president, was forced to flee to the southern port of Aden, hoping to cling to authority with the backing 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, according to security officials.

The Houthis have succeeded in their advance in large part because of help from Saleh, the autocrat who ruled Yemen for more than 30 years until he was ousted following a 2011 Arab Spring popular uprising. However, he remained in the country and some of the strongest military units remained loyal to him, undermining Hadi. Those units have now fought alongside the Houthis.

Three Egyptian military and security officials told The Associated Press that a coalition of countries led by Egypt and Saudi Arabia will conduct a ground invasion into Yemen once the airstrikes have sufficiently diminished the Houthis and Saleh's forces. They said the assault will be by ground from Saudi Arabia and by landings on Yemen's Red and Arabian Sea coasts.

The aim is not to occupy Yemen but to weaken the Houthis and their allies until they enter negotiations for power-sharing, the officials said.

They said three to five Egyptian troop carriers are stationed off Yemen's coasts. They would not specify the numbers of troops or when the operation would begin. They spoke on condition of anonymity because they were not authorized to talk about the plans with the press.

Egypt's presidency said in a statement Thursday that its naval and air forces were participating in the coalition campaign already. Egypt is "prepared for participation with naval, air and ground forces if necessary," Foreign Minister Sameh Shukri said at a gathering of Arab foreign ministers preparing for a weekend Arab summit in the Egyptian resort of Sharm el-Sheikh.

In the air assault that began Thursday -- codenamed "Operation Decisive Storm -- Saudi Arabia deployed some 100 fighter jets, 150,000 soldiers and other navy units, Saudi-owned Al-Arabiya TV reported. Also involved in the air operation were aircraft from the United Arab Emirates, Bahrain, Qattar, Kuwait, Jordan, Morocco, Sudan and Egypt, though it was not clear which carried out actual strikes.

The Arab Summit starting Saturday is expected to approve the creation of a new joint Arab military force to intervene in regional crises. The Egyptian security and military officials said the force is planned to include some 40,000 men backed by jet fighters, warships and light armor. Hadi is expected to attend the summit.

Egypt and Saudi Arabia have both intervened in Yemen before -- and struggled in its mountainous, difficult terrain. In the 1960s, Egypt sent its military to back republican forces against a Shiite royal dynasty backed, ironically in current light, by Saudi Arabia. More recently, Saudi Arabia struck against the Houthis by air and with a limited border excursion in late 2009-early 2010, a foray that killed more than 130 Saudi troops.

Support for the Houthis is far from universal in Yemen -- but foreign intervention risks bringing a backlash.

On Thursday, thousands gathered outside Sanaa's old city in the Houthi-organized protest, chanting against Saudi Arabia and the United States.

Khaled al-Madani, a Houthi activist, told the crowd that "God was on the side of Yemen." He blasted Saudi Arabia saying it is "buying mercenaries with money to attack Yemen. But Yemen will, God willing, will be their tomb."

Anger against the strikes was already brewing -- particularly after airstrikes targeting an air base near Sanaa's airport flattening half a dozen homes in an impoverished neighborhood and killing at least 18 civilians, according to the health ministry.

TV stations affiliated with the rebels and Saleh showed the aftermath of the strikes Thursday. Yemen Today, a TV station affiliated with Saleh, showed hundreds of residents congregating around the rubbles, some chanting "Death to Al-Saud", in reference to the kingdom's royal family. The civilians were sifting through the rubble, pulling out mattresses, bricks and shrapnel.

Ahmed al-Sumaini said an entire alley close to the airport was wiped out in the strikes overnight. He said people ran out from their homes in the middle of the night, many jolted out of bed to run into the streets.

"These people have nothing to do with the Houthis or with Hadi. This is destructive. These random acts will push people toward Houthis," he said, as he waved shrapnel from the strikes.

Strikes also hit in the southern province Lahj and the stronghold of Houthis in the northern Saada province. In Sanaa, they also hit the camp of U.S.-trained Yemeni special forces, which is controlled by generals loyal to Saleh, and a missile base held by the Houthis.

The strikes also hit the al-Annad air base in the southern Lahj province. About 100 U.S. military advisers withdrew over the weekend from base, where they had been leading a drone campaign against Al Qaeda in the Arabian Peninsula, or AQAP.