Forces loyal to Yemen’s former president stormed the international airport in the southern port city of Aden on Thursday, triggering an intense, hours-long gunbattle with the forces of the current president that intensified a months-long struggle for power threatening to fragment the nation.

The fighting spread into clashes between the two sides around Aden, Yemen’s second most important city and economic hub. Amid the violence, warplanes fired three air strikes at the palace of embattled president Abd Rabbo Mansour Hadi, located on a rocky hill overlooking the Arabian Sea. The strikes caused no damage and Hadi was not present at the time, official said.

The assault in Aden was the latest against Yemen’s internationally backed president, Hadi, who was driven out of the capital, Sana’a, after a takeover by Shia rebels known as Houthis. Hadi and his allies fled to Aden, which they have made their de facto capital while the rebels have solidified their hold in the north.

The attack on the airport, in which thirteen people died before eventually being fended off, was by forces loyal to Ali Abdullah Saleh, the longtime autocratic president who was ousted from power by a 2011 popular uprising. Saleh has allied with the Houthis against Hadi, and his loyalists still command parts of the military and police. The attempt to capture Aden’s airport and target the presidential palace appeared to be aimed at isolating the city and weakening Hadi’s hold.

Last week, Saleh boasted he would corner Hadi as he fled from Sanaa. “Those fleeing to the south ... will find only one exit: the Red Sea toward Djibouti,” he said in a speech to his supporters.

Tensions have been building for days in Aden. Hadi’s loyalists, in the military and police and in militias known as Popular Committees, dominate the city. But two army units in the city are loyal to Saleh, as is a force of 3,000 police special forces under a pro-Saleh commander, Brigadier General Abdul-Hafez al-Saqqaf. In a bid to strengthen his hold in Aden, Hadi tried unsuccessfully to remove Saqqaf from his post earlier this month, prompting some clashes.

The fighting started in the early hours Thursday when a unit of Saqqaf’s police special forces stormed the airport grounds, sparking heavy battles with pro-Hadi forces. Machinegun fire rang out and explosions shook the terminal building.

At least two shells hit the airport’s grounds, said security and aviation officials at the scene. Three of Saleh’s loyalists were killed and 10 were captured in the clashes, according to security and medical officials.

During the fighting, more than 100 passengers including an Associated Press reporter were rushed off a plane of the national carrier Yemenia that was on the tarmac, preparing to head to Cairo, They were rushed into the terminal building.

One of Hadi’s presidential planes, a Boeing 747, was damaged when Saleh loyalists sprayed it with gunfire, the officials said. All officials spoke on condition of anonymity because they were not authorized to talk to reporters.

During more than four hours of fighting, a convoy of tanks and armored vehicles, led by Hadi loyalist defense minister Major General Mahmoud al-Subaihi, arrived from downtown Aden to reinforce the airport’s defenders. Al-Subaihi’s troops then ordered passengers out of the terminal and the airport building, through the thick of the clashes.

Al-Subaihi’s troops succeeded in taking back control of the airport. They then turned to the adjacent base of the pro-Saleh police commandos, surrounding it and pounding it with artillery, before storming it in the afternoon, officials said.

After Saqqaf surrendered, militiamen and civilians overran the base, looting weapons and equipment. Gunmen and civilians including children carried off boxes of ammunition and rocket-propelled grenades, riot police shields and helmets and tear gas canisters, according to an AP reporter at the scene. Militiamen held machinegun posts of the defeated police and Hadi loyalists flashed V-for-victory signs from atop tanks. Several buildings outside the airport had holes from the shelling and gunfire.

The takeover of the base appeared to have prompted the airstrikes as retaliation. One or two warplanes flew over the city and fired three strikes at the palace, but did not damage it, hitting in the sea and the nearby mountains, an aide of Hadi told the AP, speaking on condition of anonymity because he was not authorized to talk to the press. Anti-aircraft fire drove off the planes.

The planes were apparently flown by loyalists of Saleh or the Houthis. In and around Sana’a in the north, the Shiite rebels have control of multiple military and air force bases and have installed loyalists in command positions, including as head of the airport.

During the day. sporadic clashes also erupted throughout Aden between Saleh and Hadi loyalists. Sounds of explosions periodically shook the city, and streets were largely deserted as residents hid in homes. By late afternoon, fighting calmed and male residents began to re-emerge onto the streets.

Armored vehicles and tanks fanned out into the streets, and fighters from Hadi’s public committee militias roamed the city in pick-up trucks mounted with heavy machine guns. They and pro-Hadi security forces deployed around hotels and government buildings and the Central Bank building. Saleh’s forces took control of the local city council building, and ambulance sirens could be heard across the city.

Yemen, the Arab world’s poorest nation, is deeply polarized and engulfed in turmoil that threatens to split the country amid the power grab by Houthis.

In January, the rebels declared themselves the country’s rulers, ordered disbandment of the parliament and formation of a presidential council in addition to a new legislative body. Hadi insists he remains the country’s legitimate leader and enjoys much international support as well as backing in the south, including Aden, where he and al-Sabaihi fled after several weeks under house arrest in the capital, Sanaa.

Meanwhile, Yemen’s al-Qaida branch, considered by Washington the terror network’s most dangerous offshoot, has profited from the turmoil and has been stepping up attacks on Yemeni forces and also the Shia rebels.

The group claimed responsibility for the assassination of a prominent Houthi supporter and politician, Abdel-Karim al-Khewani, who was gunned down in front of his house in Sana’a.