Adel al-Jubair, Saudi ambassador to the US, said on Wednesday that a coalition consisting of 10 countries, including the Gulf Cooperation Council (GCC), had begun airstrikes at 7pm Eastern time.
"The operation is to defend and support the legitimate government of Yemen and prevent the radical Houthi movement from taking over the country," Jubair told reporters in Washington.
At least 17 civilians were killed in Sanaa during the overnight offensive, civil defence sources told AFP news agency.
Huge explosions were heard in Sanaa as strikes hit the airbase at Sanaa airport and other locations in the capital, an AFP correspondent reported.
Strikes were also reported on targets in the Malaheez and Hafr Sufyan regions of Saada, a main Houthi stronghold on border with Saudi Arabia.
Citing Saudi military sources, Saudi-owned Al Arabiya TV channel reported that 100 Saudi warplanes were involved in the operation, dubbed Decisive Storm.
Kuwait’s defence ministry announced it was sending three squadrons of its F18 Super Hornet aircrafts to Saudi's King Abdulaziz airbase in Dhahran to take part in the Yemen offensive.
The United Arab Emirates were participating with 30 jets, Bahrain with eight, Morocco and Jordan both with six. Sudan reportedly offered three war planes to assist the operation, Al Arabiya reported.
Jordan confirmed to Al Jazeera that it was participating in the offensive. An Egyptian official told AFP news agency that Egypt would also take part. Saudi Arabia said that another four Muslim countries including Pakistan wanted to participate in the Saudi-led military coalition.
"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," Egypt's state news agency reported.
'Declaration of war'
Speaking to Al Jazeera from Sanaa, Houthi spokesman Mohammed Al Bukhaiti called the military action a declaration of war on Yemen, adding that reports alledging that Houthi leader Mohamed Ali Al Houthi had been injured were false.
Iranian state media called the military operation a "US-backed aggression". State television broadcast footage of some of the damage, showing dozens of bodies and some wounded people, saying "many Yemeni citizens were killed in the US-backed aggressions in Yemen".
Iran denies providing money and training to the Shia Houthi militia in Yemen as claimed by the Saudi-led coaliotn and some Western officials.
Saudi Arabia and four other Gulf states, including Bahrain, Kuwait, Qatar and the United Arab Emirates, announced earlier a decision to "answer the call of President Hadi to protect Yemen and his people from the aggression of the Houthi militia".
Military sources said rebel positions were hit at various locations in Sanaa, including at al-Daylami airbase and the adjacent international airport in north Sanaa, as well as the presidential complex seized by the rebels in January.
In the south, residents reported hearing large blasts at Al-Anad main airbase, north of Aden, which was seized by anti-government forces on Wednesday.
Three Houthi military commanders, Abdel Khaleq Badereddine Al-Houthi, Yousef Al-Madani and Yousef Al-Fishi have been reportedly killed in the attacks.
The Houthis confirmed in a statement to reporters that Saudi jets hit a military base in Sanaa, known as al-Duleimi. They said they fired anti-aircraft missiles in response.
Hakim Al Masmari, Yemen Post editor, said that "people are terrified".
"It's [bombing] not in any particular location in Sanaa, it's throughout the capital," he said.
Separately, a statement issued in Riyadh in the name of Saudi Arabia, Qatar, Kuwait, Bahrain and the United Arab Emirates - the GCC countries without Yemen's neighbour Oman - said they had been asked for help by President Abd-Rabbu Mansour Hadi's embattled government.
Al-Jubair said that for the moment the action was confined to airstrikes on various targets around Yemen, but that other military assets were being mobilised and that the coalition "would do whatever it takes".
The ambassador said he would not go into detail about the support being provided by Saudi Arabia's allies, but added "we consulted very closely with many of our allies and in particular with the United States".
"President Obama has authorised the provision of logistical and intelligence support to GCC-led military operations," National Security Council spokeswoman Bernadette Meehan said in a statement.
"While US forces are not taking direct military action in Yemen in support of this effort, we are establishing a Joint Planning Cell with Saudi Arabia to coordinate US military and intelligence support."
The Saudi ambassador said that the Houthis controlled ballistic and heavy weaponry, and could take control of the country's air force.
Yemen has been gripped by growing turmoil since Shia Houthi rebels launched a power takeover in the Yemeni capital last month.
Houthis have advanced to the southern port city of Aden, where Hadi was based after fleeing from house arrest in Sanaa.
They control much of the north, including Sanaa, and several southern provinces. In recent days, they took the third-largest city, Taiz, as well as much of the province of Lahij, both just to the north of Aden.
In fighting in Lahij, they captured Hadi's defence minister, Major General Mahmoud al-Subaihi, and then swept into the nearby al-Annad base, which the US military had left.
The strife has raised fears Yemen could be torn apart by a proxy war between Shia Iran, accused of backing the rebels - and Sunni-ruled Saudi Arabia, which supports Hadi.
Al Jazeera's correspondent Omar al-Saleh, reporting from Doha, said Iran has been expanding its influence in the region since the latest Iraq war.
"They will use their influence as a way to be taken more seriously by the US ... I think the Gulf was taken by the surprise. They were distracted by the events in Egypt and the Muslim Brotherhood."
Riad Kahwaji, a political analyst based in Dubai, told Al Jazeera that "the Houthis misunderstood the signals from the rest of the GCC and continued to take more territory".
"Saudi has three regional powers on their side. It is just the start of a combined military operation. I expect a land invasion in the coming days," Kahwaji added.
"They will also attempt to block any arms coming in from Iran. ... The people will finally see that that Arab giant (Saudi Arabia) has finally awakened."