Egypt is providing political and military support for an operation launched by Saudi Arabia and its Gulf Arab allies against Houthi fighters in Yemen, the state news agency said on Thursday.
It quoted the foreign ministry as saying coordination was under way with Saudi Arabia and other Gulf Arab countries over preparations in taking part with an Egyptian air, naval and ground force if necessary.
The announcement by Egypt "stems from its historic responsibility towards the national Arab security" and "in defense of Yemen's security and stability," the foreign ministry said.
Gulf broadcaster al-Arabiya TV reported that the kingdom was contributing as many as 150,000 troops and 100 warplanes to the operations and that Egypt, Jordan, Sudan and Pakistan were ready to take part in a ground offensive in Yemen.
Unidentified warplanes had earlier launched air strikes on the main airport in the Yemeni capital, Sanaa, and its al Dulaimi military airbase, residents said.
That came soon after Saudi Arabia's ambassador in Washington, Adel al-Jubeir, announced the operation.
"We will do whatever it takes in order to protect the legitimate government of Yemen from falling," Jubeir told a news conference.
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.
"We are very pleased with the outcome of those discussions.
"We have a situation where you have a militia group that is now in control or can be in control of ballistic missiles, heavy weapons and an air force," he said, arguing that the Houthi advance could not be tolerated.
Jubeir said that the legitimate government of Yemen was involved in a political process that had the support of the international community and that no "outside militia" would be allowed to interfere with that.
Jubeir sadded that the assaults had been launched in response to a direct request by Hadi, who supported Washington's campaign of deadly drone strikes on a powerful al Qaeda branch based in Yemen. He has been holed up in Aden with loyalist forces since he fled Sanaa in February.
Hadi remains in his base in Aden and was "in high spirits", one of his aides said after the operation began.
Washington said President Barack Obama had authorised the "provision of logistical and intelligence support" to the military action.
Yemen's slide towards civil war has made it a crucial front in mostly Sunni Saudi Arabia's rivalry with Shia Iran, which Riyadh accuses of stirring up sectarian strife throughout the region and in Yemen with its support for the Houthis.