"There is gunfire inside the airport vicinity, and we are (hearing) this is an ongoing attack involving gunmen, we don't have details so far but the security forces are engaging them now," said Mohamed Ali, who works for airport security.

Witnesses reported hearing heavy gunfire while the nation's public broadcaster Somali National Television (SNTV) said on Twitter that "security forces are attending a terrorist incident at one of main gates of Mogadishus Halane compound".

The airport complex houses the UN, aid agencies and foreign missions and contractors, as well as the headquarters of the African Union military mission, AMISOM.

"The movement of transport along the road inside the airport is stopped," Ahmed Dahir, an employee at a hotel inside the complex, said.

Al-Shabaab, which has been waging a violent insurgency against the country's fragile government for over a decade, said it carried out the attack.

The Al-Qaeda-linked militants frequent target civilian, military and government targets in the capital and outside.

The jihadists have previously claimed responsibility for mortar attacks on the airport complex, including in March last year when six rounds landed within its perimetre.

In 2020, the UN said several mortar rounds had landed in the vicinity of the UN and AMISOM compound near the airport.

Nine people, including three AMISOM soldiers, were wounded in a similar mortar attack targeting the complex in 2019.

Somalia, particularly Mogadishu, has seen a spate of attacks in recent weeks as the country hobbles through a long-delayed election process.

The president and prime minister have been at loggerheads over the vote, which is more than a year late and has been marred by violence.

After countless delays, the lower house election is now due to be completed on March 31, paving the way for lawmakers to pick a president.

The impasse has worried Somalia's international backers, who fear it distracts from the fight against Al-Shabaab.

Somalia's key foreign backer, the United States, has already imposed travel sanctions on key political figures for undermining the electoral process.

The jihadists controlled Mogadishu until 2011 when it was pushed out by AMISOM troops, but it still holds territory in the countryside.