Suicide bomb and gun attacks have hit central Kabul, with a string of blasts targeting the US embassy and one death reported in a suicide attack in the west of the Afghan capital.
Gunmen are holed up in a high-rise building overlooking the city centre, exchanging gunfire with police.
The Taliban say they are behind the attacks.
They come weeks after suicide attackers stormed the city's British Council office, killing 12 people.
The Taliban said they also carried out that hours-long attack on 19 August to mark the anniversary of Afghanistan's independence from the UK in 1919.
Co-ordinated attack
Tuesday's attack appears to be a complex operation with a number of suicide bombers targeting Kabul's upmarket embassy district, while one policeman was reportedly killed and two others injured by a suicide bomber in the west of the city.
"Today at 13:00 (08:30 GMT) at Kabul's Abdul Haq roundabout a massive suicide attack on local and foreign intelligence facilities is ongoing," said a spokesman for the Islamist group of Tuesday's attack.
Nato's Secretary General Anders Fogh Rasmussen condemned the violence, saying it was an attempt to test Afghanistan's security transition (the handover of control from Nato to Afghan-led forces) that would not succeed.
"We have confidence in the Afghan authorities' ability to deal with this situation," said Mr Rasmussen in Brussels. "Transition is on track and it will continue."
Around five attackers are coordinating their operation from the high rise building which is under construction just 300m from the US embassy, correspondents say, with a rocket-propelled grenade apparently overshooting the embassy.
US officials confirmed the attack around the embassy. "There are no casualties at this time among embassy personnel," said a spokeswoman.
US marines were seen on the roof of the embassy building assessing the situation and checking their defences were robust, correspondents say. Military helicopters are in the area.
Western security sources said Nato's International Security Assistance Force (Isaf) headquarters was also targeted, although an Isaf spokesman would not confirm the information.
Twisted metal
Casualty figures in the embassy district remain unclear. Reports say four badly wounded people have been taken to two hospitals in the area.
One witness, Mohammad Zada, said he was driving past the US embassy building when the attack started, adding that he heard five explosions.
"Nothing really prepares you for shards of twisted metal, scattered glass, victims and the debris that litters the streets," said Mr Zada, who works for Agility Global Integrated Logistics in Kabul.
Video footage showed a group of men taking shelter from gunfire - one with blood pouring from his arm - as sirens wailed through the city.
A Press TV cameraman was said to have been injured in the violence.
The Afghan security forces - both the police and a quick reaction force from the Afghan army - have sealed off streets in the upmarket district and urged residents to stay away from windows.
As well as housing the US and other foreign missions, the area is home to a number of government ministry buildings and the presidential palace.
This year has seen the most bloodshed in Afghanistan since US-led forces toppled the Taliban in 2001.
The violence has intensified since July, when Nato began the long process of handing power over to Afghan forces. Most international troops are scheduled to leave by 2015, provided Afghan forces are ready to take over security.
