Saudi and Egyptian warships have deployed to the Bab al-Mandab strait off Yemen to secure the strategic sea passage, Egyptian military officials said, as a Saudi-led coalition widened air strikes against Shiite rebels and their allies
The air strikes continued for a second day, aiming to pave the way for possible ground operations.
A priority after the air campaign has weakened the rebels is for coalition troops -possibly Egyptians - to move into the southern port city of Aden, a main stronghold of supporters of President Abed Rabbo Mansour Hadi, who was forced to leave the country as the rebels and their allies moved on Aden, Yemeni and Egyptian military officials said.
That could prove a tough prospect. Today, rebel fighters and their allies - military units loyal to former president Ali Abdullah Saleh - were moving through southern Abyan province toward Aden, aiming to reinforce their fighters already in the city, Yemeni security officials said.
At the same time, pro-Hadi military units and militiamen were fighting rebel forces in street battles in several southern cities.
The conflict abruptly burst into a regional fight yesterday after months of chaos within Yemen.
Saudi Arabia and its allies are aiming push back the Shiite rebels, known as Houthis, and Saleh's forces, who have taken over the capital Sanaa and much of the mountainous, impoverished nation.
The ultimate goal is to restore Hadi, who is expected to arrive in Egypt for an Arab summit tomorrow. Saudi Arabia fears the Houthis will give Shiite powerhouse Iran a new foothold on its southern border.
Yemeni foreign minister Riad Yassin said there was an "arrangement" for ground troops of the Saudi-led coalition to deploy in Yemen. "It's a comprehensive military operation," he told the Saudi-owned Al-Arabiya news channel. He said Egyptian naval forces are heading to Bab al-Mandab.
Several Egyptian military officials said Egyptian and Saudi warships were already at or near the strait. Egypt has said securing the passage is a priority as Bab al-Mandab is the entrance to the Red Sea, leading from the Arabian Sea to its Suez Canal, a vital route for shipping between Europe and Asia.
After more than 36 hours of air strikes, more than 40% of Yemen's air defences were destroyed, according to Yemeni Brigadier General Saleh al-Subaihi, a pro-Hadi officer.
Yemeni security officials said around 80 fighters had been killed in the strikes - some from the Houthis, but most from Saleh's forces. The first salvo of air strikes before dawn yesterday also killed 18 civilians - including six children - when they flattened a block of homes in an impoverished neighbourhood near Sanaa's airport.
Diminishing Saleh's forces is a key step before any ground action. Saleh, who ruled Yemen with an autocratic hand for decades, was removed in 2012 following an Arab Spring popular uprising against him, and Hadi was installed in his place.
But Saleh remained in Yemen and maintained widespread influence, particularly in the fragmented military and security forces. One Yemeni security official estimated that 70% of the military remained loyal to him, including many of the best trained and equipped units, and they are in bases around the country.
Their support for the Houthis has been crucial to the rebels' takeover.
Air strikes today struck in at least six provinces. In the capital Sanaa, heavy strikes came in waves overnight, shaking the city as anti-aircraft guns fired.
New strikes hit Saada, the northern stronghold of the Houthis, aimed at locations where rebel leader Abdul-Malek al-Houthi might be, the Yemeni military officials said.
The grave of his brother Hussein al-Houthi, founder of the rebel group, was demolished in the strikes. The attacks prompted Houthis to shut down schools and cancel classes indefinitely, according to a statement sent to reporters by the group.
Saudi planes also bombed camps and bases of pro-Saleh army forces north east of Sanaa and in the southern provinces of al-Dhale and Lahj. In the oil and gas-rich north-central provinces of Marib, strikes targeted radar facilities.
Retired Yemeni army officer Nasser al-Marqashi said he expects the air strikes to continue for a week to weaken air defences before a ground offensive, which would probably be launched from Aden or from the country's sparsely populated far eastern Hadramout province, where Hadi also has supporters.
Houthis and the Saleh loyalists control at least 10 of Yemen's 21 provinces, but in many of those places, their hold is not complete, particularly in the south. In Dhamar and Taiz - two areas overrun by the rebels - thousands of demonstrators staged protests in support of the Saudi air strikes.
In the southern city of al-Dhale - capital of the province of the same name - pro-Hadi militias were fighting Saleh's forces.
Battles were also going on in the southern city of al-Houta, north of Aden. Part of the city is controlled by pro-Hadi militias, another part is controlled by al Qaida militants, and both were separately fighting Houthi and pro-Saleh forces trying to take the area, Yemeni security officials said.
Saudi Arabia and fellow Sunni-led allies in the Gulf and the Middle East view the Houthi takeover as an attempt by Iran to establish a proxy on the kingdom's southern border. Iran and the Houthis deny that Tehran arms the rebel movement, though it says it provides diplomatic and humanitarian support.
According to defence officials in Washington, the US is providing refuelling tankers and surveillance flights for the Saudi operations, and there are several US troops working in the operations centre. The White House said the US was not taking direct military action.
Iran has denounced the Saudi-led air campaign, calling it "dangerous step".
Foreign minister Mohammed Javad Zarif said in a statement that Tehran was "ready to co-operate with its brothers in the region, to make it easier for different groups in Yemen to have dialogue to protect the (country's) integrity and facilitate restoration of stability".
The comment came after Turkish president Recep Tayyip Erdogan said in an interview that his country could provide logistical support for the Saudi-led military operation.
Saudi Arabia praised the US for helping the air campaign.
The Saudi ambassador in Washington, Adel al-Jubeir, said the US had been "very supportive" of the operation politically and logistically. He says the kingdom was "very pleased" with the level of co-ordination with the US.
Mr al-Jubeir spoke after meetings with State Department officials.
He said the mission was clear: Support Yemen's legitimate government, protect civilians and prevent militants armed with heavy weapons, ballistic missiles and air power from "controlling, dominating and damaging Yemen and its people".