Sanaa, Yemen: Saudi Arabia bombed the northern stronghold of Yemen’s Shiite rebels and key military installations on Friday, the second day of a campaign that military officials said was aimed at neutralising air defences ahead of a possible ground invasion.

The intervention that began early Thursday is aimed at halting the advance of the Iran-backed rebels, known as Al Houthis, who have allied with a powerful former president and seized the capital and several provinces.

The air strikes have struck at least six provinces, including one rich in oil and gas, and opened a new front in the regional rivalry between Riyadh and Tehran. Yemeni President Abd Rabbo Mansour Hadi, a close US ally, has fled the country, and the turmoil could bolster a powerful local Al Qaida affiliate as well as militants inspired by Daesh.

The air strikes have destroyed nearly 40 per cent of Yemen’s air defences, including radar installations, according to Yemeni Brig. Gen. Saleh Al Subaihi. The strikes have also hindered the ability of the Houthis and military units loyal to former President Ali Abdullah Saleh to send reinforcements to the main front in the south, he said.

Retired army officer Nasser Al Marqashi said he expects the air strikes to continue for a week to weaken the air defences before a ground offensive, which would likely be launched from the port city of Aden, former capital of the once-independent south, or the country’s sparsely populated far east. Hadi has supporters in both areas.

Three Egyptian security officials, who spoke on condition of anonymity because they were not authorised to discuss future operations with the press, said troops would invade from Saudi Arabia and land on Yemen’s coast.

The air strikes on early Friday targeted Saada, the northern stronghold of the Houthis, and were aimed at locations where rebel leader Abdul Malek Al Houthi might be, military officials said. The grave of his brother Hussain Al Houthi, founder of the rebel group, was demolished in the strikes.

Saudi warplanes also bombed the Al Sana army camp northeast of Sanaa, run by commanders loyal to Saleh. An autocrat who ruled Yemen for more than 30 years until he stepped down following a 2011 anti-government protest, Saleh enjoys the loyalty of some of the country’s strongest military units, who now fight alongside the Houthis.

The officials said other strikes targeted army camps outside Sanaa in Al Dhale and Lahj provinces, including Al Annad base, where about 100 US military advisers had been based, but were forced to withdraw last weekend due to deteriorating security conditions.

In oil and gas-rich Marib, strikes destroyed radar that the rebels could use to track Saudi warplanes.

In Dhamar and Taiz — two areas overrun by the rebels — thousands of demonstrators staged protests in support of the Saudi air strikes.

In the capital Sanaa, heavy air strikes came in waves overnight. Explosions rocked the city and anti-aircraft guns were heard returning fire. At least 18 civilians, including six children, have been killed and the casualty count was expected to rise.

Saudi Arabia and fellow 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.

Yemeni Foreign Minister Riad Yassin told Saudi-owned Al Arabiya TV that the offensive is also meant to stop outside supplies coming from Iran. “We have a lot of evidence that there is a group from the Iranian Revolutionary Guard and foreign experts,” heading to Sanaa, he said. The Houthis have denied such allegations in the past.

Iran has denounced the Saudi-led air campaign, calling it “dangerous step.”

Iranian Foreign Minister Mohammed Javad Zarif said in a statement on Friday that Tehran was “ready to cooperate 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. Zarif said Iran respects its strategic relations with Turkey.

In the air assault, code-named ‘Storm of Resolve’, Saudi Arabia deployed some 100 fighter jets, 150,000 soldiers and other navy units, Al Arabiya reported. Also involved were aircraft from the UAE, Bahrain, Qatar, Kuwait, Jordan, Morocco, Sudan and Egypt, though it was not clear which carried out the actual strikes.

On Friday, Egypt’s President Abdel Fattah Al Sissi reiterated Cairo’s commitment to Gulf security in a phone call with Saudi King Salman, saying “the security of the Gulf is a red line and part and parcel of Egyptian national security,” according to presidential spokesman Alaa Youssef.

Yassin, the Yemeni foreign minister, said Egyptian naval forces are heading toward the Bab Al Mandab strait, the narrow opening to the Red Sea.

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.

Al Houthi, the leader of the rebels, has accused the US, Saudi Arabia and Israel of launching a “criminal, unjust, brutal and sinful” campaign aimed at invading and occupying Yemen.

“Yemenis won’t accept such humiliation,” he said in a televised speech on Thursday night, calling the Saudis “stupid” and “evil.”

President Hadi fled the country by boat on Wednesday and arrived in Riyadh the following day after transiting through Oman. He was expected to attend a meeting of Arab leaders this weekend in the Egyptian resort town of Sharm Al Shaikh.

Brig. Gen. Ahmad Asiri, the spokesman for ‘Storm of Resolve’, denied that a ground assault was part of the current operation, but said the coalition was ready “to deter any aggression of any kind.”

He told reporters in Saudi Arabia on Thursday that the air strikes were destroying Houthi air defence systems and targeting bases, on-the-ground aircraft and ballistic missiles. Within the first 15 minutes of air strikes, the Saudis had “complete control on the skies of Yemen,” he said.

Each air sortie was led by Saudi warplanes and included several other support aircraft with teams for refuelling, early warning, reconnaissance and search and rescue.

United Nations: Saudi Arabia and its Gulf allies presented powerful members of the UN Security Council with suggested elements for a resolution that would impose an arms embargo on Al Houthi rebels who have thrown Yemen into turmoil and caused its Western-backed president to flee.

Diplomats said after a meeting on Thursday evening with ambassadors from the US, Britain and France that members of the Gulf Coordination Council (GCC) want the resolution to be militarily enforceable under Chapter seven of the UN Charter.

The GCC members also presented the Security Council president with a letter explaining that their air strikes targeting military installations held by the Al Houthi rebels are in response to President Abd Rabbo Mansour Hadi’s request this week for immediate aid as the rebels closed in.

The current president of the Security Council, French Ambassador Francois Delattre, did not comment on his way out of the meeting. Another permanent council member, Russia, met with the Gulf members earlier on Thursday.

One diplomat who attended the meeting said the GCC members stressed that an arms embargo would block arms only to the Al Houthi rebels and not to Yemen’s government. The diplomat said the members do not want a repeat of Libya, whose internationally recognised government has to ask a UN sanctions committee for exemptions to that country’s arms embargo when it wants to obtain weapons.