Deadly clashes forced the closure of the international airport in Yemen's main southern city Aden on Thursday, where the conflict-ridden country's leader fled after militia forces seized the capital.

At least six people were killed and 20 wounded after fierce fighting broke out between supporters and opponents of President Abedrabbo Mansour Hadi, security sources said.

The clashes erupted between a special forces unit loyal to the Shiite Huthi militia who have seized control of the capital and pro-Hadi paramilitary fighters, security sources said.

The special forces, led by renegade General Abdel Hafez al-Saqqaf, met with heavy resistance from fighters with the Popular Committees, a local paramilitary group that backs Hadi.

Several hundred soldiers backing Hadi -- supported by tanks and armoured vehicles -- were also dispatched as reinforcements to the airport.

Three members of the special forces were killed and two from the Popular Committees, sources said. Another special forces member was killed in separate fighting in central Aden.

The Hadi loyalists regained control of the airport early Thursday after several hours of fighting, a military source told AFP.

Yemen, a key US ally in the fight against Al-Qaeda, has been gripped by unrest since ex-strongman Ali Abdullah Saleh stepped down in early 2012 after a year-long popular uprising against his rule.

Saleh -- along with Iran -- has been accused of backing the Huthis, who took control of Sanaa in September and earlier this year seized key government buildings in the capital.

Hadi resigned after the rebels seized his palace and named a "presidential council" to rule the country and assume the powers of the presidency.

Hadi was placed under house arrest in Sanaa but later resurfaced in Aden and retracted his resignation, despite being increasingly isolated in his southern Yemen power base.

Washington fears that the ongoing political instability will help Al-Qaeda in the Arabian Peninsula (AQAP), considered the jihadist network's most dangerous branch, to flourish.

During the fighting on Thursday shots were fired at the airport's control tower, witnesses said.

A military source told AFP that "Saqqaf's troops were forced to retreat to their camp (north of the airport) after being subjected to heavy shelling".

Saqqaf has defied a decree by Hadi sacking him and said he will only follow orders from the presidential council in Sanaa.

Flights were cancelled due to the clashes and had not resumed by midday Thursday.

One passenger who went to the airport said he had to turn back.

"I could not move. All access to the airport was blocked by armed men," he told AFP.

Other clashes were reported away from the airport including close to local government buildings in the centre of Aden and the local branch of the Central Bank.