SANAA, Yemen (AP) — Saudi Arabia launched airstrikes Thursday targeting military installations in Yemen held by Shiite rebels who were taking over a key port city in the country's south and had driven the embattled president to flee by sea, security officials said. Some of the strikes hit positions in the country's capital, Sanaa.

The airstrikes, which had the support of nine other countries, drew a strong reaction from Iran which called the operation an "invasion" and a "dangerous step" that will worsen the crisis in the country.

The back-and-forth between the regional heavyweights was threatening to turn impoverished Yemen into a proxy battle between the Middle East's Sunni powers and Shiite-led Iran.

Saudi-owned Al-Arabiya News reported that the kingdom had deployed 100 fighter jets, 150,000 soldiers and other navy units.

The Shiite rebels, known as Houthis, were calling on their supporters to protest in the streets of Sanaa on Thursday afternoon, Yemen's Houthi-controlled state news agency SABA reported.

On Wednesday President Abed Rabbo Mansour Hadi, a close U.S. ally, fled Yemen by sea as the rebels started taking over the southern port city of Aden where he had taken refuge.

Saudi ambassador to the United States Adel al-Jubeir announced the military operation in a news conference in Washington. He said his government had consulted closely with the U.S. and other allies but that the U.S. military was not involved in the operations.

The White House said in a statement late Wednesday that the U.S. was coordinating military and intelligence support with the Saudis but not taking part directly in the strikes.

Other regional players were involved in the Saudi operation: The United Arab Emirates, Kuwait, Qatar and Bahrain joined Saudi Arabia in a statement published by the Saudi Press Agency, saying they would answer a request from Hadi "to protect Yemen and his dear people from the aggression of the Houthi militias which were and are still a tool in the hands of foreign powers that don't stop meddling with the security and stability of brotherly Yemen." Oman, the sixth member of the Gulf Cooperation Council, didn't sign onto the statement.

Egypt also announced political and military support. "There is coordination ongoing now with Saudi Arabia and the brotherly gulf countries about preparations to participate with an Egyptian air and naval forces and ground troops if necessary," it said in a statement carried by the state news agency.

Pakistan, Jordan, Morocco and Sudan were also joining the operation, the Saudi Press Agency reported Thursday.

Saudi Arabia and its Gulf allies believe the Houthis are tools for Iran to seize control of Yemen and say they intend to stop the takeover. The Houthis deny they are backed by Iran.

Security officials in Yemen said the Saudi airstrikes targeted a camp for U.S.-trained special forces, which is controlled by generals loyal to former President Ali Abdullah Saleh. The officials said the targets included the missile base in Sanaa that was controlled by the Houthis earlier this year. One of the Yemeni security officials said the strikes also targeted the fuel depot at the base.

The Houthis said in a statement to reporters that Saudi jets hit the military base, known as al-Duleimi, and that they responded with anti-aircraft missiles.

Riad Yassin, Yemen's foreign minister, told Saudi's Al-Hadath TV that the airstrikes were welcomed.

"I hope the Houthis listen to the sound of reason. With what is happening, they forced us into this," he said.

The crumbling of Hadi's government is a blow to Washington's counterterrorism strategy against al-Qaida's branch in Yemen, considered to be the most powerful in the terrorist network. Over the weekend, about 100 U.S. military advisers withdrew from the al-Annad air base where they had been leading a drone campaign against al-Qaida in the Arabian Peninsula, or AQAP.

Yemen now faces fragmentation, with Houthis controlling much of the north, including the capital of Sanaa, and several southern provinces. In recent days, they took the third-largest city, Taiz, as well as much of the province of Lahj, both just to the north of Aden.

The Houthis are backed by Saleh, the autocrat who ruled Yemen for three decades until he was removed amid a 2011 Arab Spring uprising. Some of the best-equipped and trained military and security units remained loyal to Saleh and they have helped the Houthis in their rapid advance.

Hadi left Sanaa for Aden earlier this month after escaping house arrest under the Houthis, who overran the capital six months ago. In Aden, he had sought to make a last stand, claiming it as the temporary seat of what remained of his government, backed by allied militias and loyal army units.

With Houthis and Saleh forces closing in on multiple fronts, Hadi and his aides left Aden Wednesday on two boats in the Gulf of Aden, security and port officials told The Associated Press. The officials would not specify his destination.