As Western diplomats and staff fled Yemen on Wednesday, concern widened over the increasing turmoil in the impoverished nation, with Saudi Arabia arming loyal tribesmen across its southern border and Egypt readying a military unit to intervene if needed.
The US, British and French moved to close their embassies, signaling a belief that conditions in Yemen would only deteriorate further as the rebels, who have taken over in nearly half the provinces, try to expand their control.
In a show of bravado, the rebels seized about 20 vehicles left by US diplomats and Marines at Sanaa's airport, officials said, speaking on condition of anonymity because they were not authorized to talk to the press. The Americans evacuated after destroying documents and heavy weapons at the embassy.
The rebels also seized weapons found in the US vehicles, the officials added — apparently referring to personal sidearms that the Pentagon said the Marines left behind because they could not take them on their departing commercial flight.
The Marine Corps said in a statement Wednesday that those sidearms were destroyed with sledgehammers at the airport before the Marines departed.
While Yemen has been in chaos for years, events took a dramatic new turn when the rebels, known as Houthis and suspected of being backed by Iran, took over the capital last fall and have spread over more of the country.
In January, the rebels put US-backed President Abed Rabbo Mansour Hadi and his Cabinet ministers under house arrest, leading to their resignations. Subsequently, the Houthis, who are followers of the Shiite Zaydi sect in Sunni-majority Yemen, dissolved parliament and declared they were taking over the government.
The turmoil is starting to resonate around the Middle East, already shaken by bloody conflicts in Syria, Libya and Iraq.
As Houthi fighters advance to take more ground, Yemeni officials said Saudi Arabia, a staunch US ally, was sending arms and funds to tribesmen in Yemen's Marib province to bolster them against the rebels.
Saudi Arabia has in recent months repeatedly stated its concern over the Houthis' power grab, but the deeply secretive oil-rich kingdom has said nothing about arming or funding tribesmen there to fight the Shiite rebels.
Marib is an exclusively Sunni, energy-rich desert area on the border with Saudi Arabia where tribes have long been close to the Saudis. It is also home to a sizable number of militants from the local branch of al-Qaida, the Houthis' sworn enemy.
Marib's tribal leaders, like many others in Yemen, have been on the receiving end of Saudi largesse for decades, and some of them hold Saudi nationality.
"Marib is the heart of Sunni tribal power," said Majid al-Modhaj, a Yemeni analyst. "Fighting there will take the Houthis away from their comfort zone in mountainous areas and into plain and flat desert land they are not used to."
Egypt has set up a special rapid deployment force that could intervene if the Houthis threaten shipping lanes in the strategic Red Sea, according to three Egyptian security officials. The force, they said, is drawn from the 3rd Army, which has been running security and intelligence operations in the Red Sea from its headquarters in Suez.
Yemen lies on one side of Bab al-Mandab, the narrow southern entrance of the Red Sea. The corridor leads up the Egyptian and Saudi coasts to Egypt's Suez Canal, a key sea route for oil traffic from the Gulf region.
The Egyptians and Saudis were coordinating a joint military response to deal with any eventuality in Yemen, including the disruption of shipping, the officials said.
The officials in both Yemen and Egypt spoke on condition of anonymity because they were not authorized to talk to the media.
"Yemen is like the moon to Egypt, but it is important because of Cairo's close ties with Saudi Arabia, to whom Yemen is a priority issue," said Michael W. Hanna, a Middle East expert from the New York-based Century Foundation.
Egypt and Saudi Arabia have forged close military ties since Egyptian President Abdel-Fattah el-Sissi took office in June, with frequent joint war games, including naval exercises in the Red Sea. Thousands of Egyptian special forces are embedded with their Saudi counterparts on the kingdom's border with Iraq as a precaution against militants of the extremist Islamic State group, according to the officials.
As the region's two most powerful Sunni nations, Saudi Arabia and Egypt view the rise of the Houthis with alarm, seeing them as a new geopolitical triumph by non-Arab Iran after it consolidated its influence in Iraq, Syria and Lebanon.
The Houthis deny links to Iran, and it has been difficult to determine with any accuracy Tehran's role in the latest events.
Still, Iranian President Hassan Rouhani and a senior Foreign Ministry official made it clear in separate comments Wednesday that the Islamic Republic looks approvingly at events in Yemen.
"The power that assisted the people of Syria, Iraq, Lebanon and Yemen against terrorist groups was the Islamic Republic of Iran," Rouhani told a large crowd in Tehran. He did not elaborate.
Deputy Foreign Minister Hossein Amir Abdollahian said the developments in Yemen "have increased stability in the region and made the situation difficult for terrorists in that country." The chief of staff of Iran's military, Gen. Hassan Firouzabadi, said Sanaa was now "one of the safest places in the region" after the Houthi takeover.
Saudi Arabia and Egypt are not new to military involvement in Yemen.
Saudi Arabia fought a brief border war against the Houthis in 2010 to halt incursions over the frontier. Egypt in the early 1960s deployed thousands of troops in Yemen to support a republican coup that toppled a monarchy subscribing to Zaydi Shiism, like the Houthis.
Houthi rebels seized the province of Bayda, south of Sanaa, on Tuesday with help from government forces still loyal to Ali Abdullah Saleh, the autocratic president who was ousted in the country's 2011 Arab Spring uprising.
Bayda is widely viewed as the gateway to the country's south, but taking over that region is unlikely to be easy.
Hadi — a southerner — commands armed militias that fought al-Qaida militants in the province of Abyan in 2011 and 2012. Moreover, a key political faction in the south, the Nasserists, have close ties with Egypt, whose intelligence and security agencies have stepped up their activities in the south in anticipation of a Houthi attempt to capture the region, according to the Egyptian officials.
The Houthis' advances are also fueling secessionist movements in the south, once a separate nation.
"They won't have a friendly environment in the south," said Baraa Shiban, a Yemeni analyst. "Any attempt by the Houthis to take over the south will lead to secession."
Houthis have captured territory largely because of deals with provincial powers and massive help from army and police units loyal to Saleh. Effective battlefield resistance against their advances might finally come in Marib or in the south.
"The Houthis are spoiling for a fight, thinking that a battlefield victory will grant them a measure of legitimacy," said Sarah Gamal, a Yemeni political activist. "So far, they have just been assaulting peaceful protesters in Sanaa and elsewhere who reject their rule."