|UN officials said there were no casaulties in the attack by suicide bombers [EPA]
The United Nations office in the western Afghan city of Herat has come under attack from suspected suicide bombers and gunmen, an official of the organisation said.
"The incident is ongoing. We are working closely with local authorities," Dan McNorton, a media officer for the UN in the Afghan capital Kabul, told the AFP news agency on Saturday.
The attack, for which the Taliban claimed responsibility, began at 12.30pm (08:00 GMT).
A reporter for The Associated Press news agency said the entrance to the office had been destroyed in an explosion and that Afghan and Nato troops had surrounded the area.
Al Jazeera's James Bays in Kabul said it is "still a very confused picture" and that the incident had been going on for two hours.
"It seems likely that there are gunmen inside the main UN compound," he said.
"This is what we know about what happened: We understand that there was a suicide bomber who detonated his device outside the main gate of the UN compound and then after that, we understand, other gunmen entered the building some of them, we believe, may be equipped with suicide vests and ... weapons."
He said the police "managed to shoot one of the attackers who was dressed ... in a woman's burqa".
"The UN are telling us that they don't know the current situation inside the building, but they believe there may well be these attackers still alive inside the building," Bays said.
A UN official who asked not to be named said there had been no reports of any casualties.
"We're working closely with local police and security," he said.
The Taliban, known for routinely exaggerating the scale and impact of their operations, said 12 guards of the UN Assistance Mission in Afghanistan, or Unama, had been killed in the attack.
But our correspondent said UN employees did not sufer any casaulties.
Last October, Taliban fighters attacked a UN guesthouse in Kabul, killing five foreign UN staff. That attack prompted the United Nations to evacuate hundreds of foreign workers.
Herat, Afghanistan's second biggest city, is near the border with Iran and has until recently been relatively peaceful.
However, residents have said that the road to the local airport is controlled after dark by criminal gangs and some districts of the city, notably those dominated by ethnic Pashtuns, are reportedly controlled by Taliban fighters.
Al Jazeera and agencies