Violence has surged across the country since early May when the Taliban launched a sweeping offensive as the US-led foreign forces began a final withdrawal which is now almost complete.

The militants have seized scores of districts across the country, including in Herat province, where the group has also captured two border crossings adjoining Iran and Turkmenistan.

On Friday, the Taliban clashed with government forces on the outskirts of Herat city, the provincial capital, forcing scores of families to flee, residents said, as the insurgents tightened their noose.

During the fighting the UN's main compound in Herat came under attack by rocket-propelled grenades and gunfire, a statement issued by the United Nations Assistance Mission in Afghanistan (UNAMA) said.

"This attack against the United Nations is deplorable and we condemn it in the strongest terms," said Deborah Lyons, the United Nations Secretary-General's Special Representative for Afghanistan.

"The perpetrators of this attack must be identified and brought to account."

UNAMA said the attack was carried out by "anti-government elements".

It said, however, that the area where the compound is located was the scene of heavy fighting between the Taliban and government forces.