Insurgents launched a pre-dawn attack on Afghanistan's main international airport in the capital, Kabul, on Monday, police said, with explosions and gunfire heard coming from an area that also houses major foreign military bases.
There were no immediate reports of casualties and there was also no early claim of responsibility for the attack.
Attacks on the heavily guarded airport, used by civilians and the military, are relatively rare and would represent an ambitious target for insurgents, with recent assaults staged against less well-protected targets.
The airport, by comparison, is home to a major operational base for NATO-led forces that have been fighting Taliban and other insurgents for 12 years and is bristling with soldiers and police, guard towers and several lines of security checkpoints.
Police said the attack appeared to be centred on the military side of the airport, to the west of the civilian terminal.
"Gunmen have entered a house under construction in the west of Kabul airport and are fighting with security forces," Kabul police spokesman Hashmatullah Stanekzai said.
"Their target is Kabul airport and all roads to it are sealed," he said.
A spokesman for the Afghan Air Force, which is also based at the facility, also said the airport was the target of the attack. There are also a number of logistics bases in the area.
The attack began at about 4.30 a.m. (2400 GMT). Embassies in the diplomatic zone in the centre of Kabul were quickly locked down and emergency alarms were heard ringing loudly from the British embassy.
Reuters witnesses reported hearing explosions at the airport, with reports of rocket-propelled grenades and gunfire. Blasts still being heard an hour after the attack was launched.
Concerns are mounting over how the 352,000-strong Afghan security forces will cope with an intensifying insurgency once most foreign combat troops leave Afghanistan by the end of 2014.
The airport attack came soon after assaults on the International Organisation for Migration in Kabul and against the International Committee of the Red Cross in the eastern city of Jalalabad.
Four people were killed and three wounded in those attacks.
In April 2011, a rogue Afghan air force officer shot and killed eight U.S. servicemen and a civilian contractor in the worst attack at the airport since the war began.
Additional reporting by Dylan Welch and Omar Sobhani; Writing by Dylan Welch; Editing by Paul Tait