Foreigners are escorted to safety following an attack by militants in Kabul that killed six UN workers on Wednesday. (Mohammad Ishaq/Reuters)

Twelve people, including six United Nations staff members, were killed in Afghanistan in an attack on a UN guest house that appears to be part of an effort by militants to undermine next month's presidential run-off election.

The deaths occurred as gunfire erupted at the guest house Wednesday after dawn in the Shar-e-Naw district of Kabul.

In this image from APTN television after the attack, black smoke billows from a building used by the UN in Kabul. (APTN/Associated Press)

UN spokesman Adrian Edwards confirmed that six UN employees were killed and nine others were wounded. Twenty UN staff were known to be registered at the guest house, Edwards said, but he was unsure whether all were there at the time of the attack.

He did not know the workers' nationalities, but said they were non-Afghans. Many of the UN workers staying in the guest house were election workers, according to reports.

U.S. officials have confirmed that one American is among the dead UN employees.

The UN "will never be deterred by terrorist attacks," Secretary General Ban Ki-moon told reporters in New York on Wednesday.

The UN will continue its preparations ahead of the Nov. 7 election and is making arrangements to further ensure the safety of its staff, Ban said.

Three attackers, two security guards and an Afghan civilian also died.

Attack continued for hours

Witnesses said the attackers appeared to be wearing police uniforms when they stormed the building. Victims were screaming amid the gunfire and some were jumping from windows of the three-storey building, witnesses said.

Nearby residents reported hearing gunfire and explosions coming from the building for about two hours.

A fire was also started in the building but it is unclear how. Some witnesses reported seeing attackers who appeared to be wearing suicide vests.

Survivors of the attack told reporters that a group of guests hid in a laundry room at the back of the building before escaping over a back wall. Others reported having stayed in their rooms, which were filling with smoke, while calling people outside the building for help.

President Hamid Karzai condemned the attack as "an inhuman act" and called on the army and police to strengthen security around all international institutions. The guesthouse attack was the third major assault in the capital in recent weeks.

'Very serious incident'

"This has clearly been a very serious incident for us," Edwards said. "We've not had an incident like this in the past."

An Afghan police officer carries a wounded man away from the site of the attack. (Reuters)

Taliban spokesman Zabiullah Mujahid claimed responsibility for the attack and said it was related to the upcoming run-off in the Afghanistan election. Mujahid told The Associated Press that three militants with suicide vests, grenades and machine-guns carried out the assault.

Mujahid said the Taliban had issued a statement days ago threatening anyone working on the Nov. 7 run-off election between Karzai and Abdullah Abdullah.

"We have said that we would attack anyone engaged in the process and today's attack is just a start," Mujahid told Reuters.

After the attack, a rocket slammed into the grounds of the luxury Serena Hotel. The device did not explode but filled the lobby with smoke, forcing guests and employees to flee to the basement, according to an Afghan witness. A rocket also struck the outer limit of the presidential palace. There were no casualties.

"Certainly one of the aims of the Taliban attack today was to show that they are a force that can disrupt the poll," Afghan analyst Qaseem Akhgar told Reuters.

Afghans vote Nov. 7 in a second-round election after UN-backed auditors threw out nearly a third of Karzai's votes from the Aug. 20 election after finding widespread fraud.

Canadian Foreign Affairs Minister Lawrence Cannon condemned the attack as "cowardly" but said it will not deter the international community from supporting Afghans as they exercise their right to vote.

The Taliban have warned Afghans to stay away from the polls or risk attacks. Dozens of people were killed in Taliban attacks during the August balloting, helping drive down turnout.

With files from The Associated Press