At least 13 people, including one Briton, have been killed in Kabul after three Taliban suicide bombers attacked a Lebanese restaurant popular with foreigners and wealthy Afghans in the heart of the capital's heavily fortified diplomatic quarter.

The UN said that four of its civilian personnel who may have been near the scene of the attack still remain unaccounted for. The UN is making efforts to clarify the status of its personnel.

The Foreign Office in London confirmed that a British citizen had been killed in the attack.

The attack on a relatively unprotected restaurant was a clear statement of insurgent intent in a year critical for Afghanistan's future.

A presidential election in April will select the country's first new ruler in more than a decade, and foreign combat troops finish their mission, leaving the Afghan police and army to battle the Taliban alone.

Friday's attack began with a bomb blast that shook the Wazir Akbar Khan neighbourhood shortly after 7pm, as one attacker apparently detonated his explosives at the restaurant's fortified gate. It was followed by more than two hours of sporadic gunfire, as Afghan commandos besieged two other attackers apparently holed up inside. Mohammad Zahir, the Kabul police chief, said at least 13 people had died, with several others injured. Afghans and foreigners were among the dead, he said, but declined to give further information. "Damn! Never experienced so close an attack. Shootings and screams of horror in the street. Broken windows in our house!" one Afghan who lives near the restaurant posted on Facebook.

The Taliban claimed responsibility, saying they had targeted "foreign invaders" and there were Germans among the dead.

Although there are regular attacks on targets in the Afghan capital, it is rare for would-be attackers to make it through rings of security around the city, elude an extensive intelligence network, and strike with such deadly force at a civilian target around the heavily fortified "green zone" that houses Nato and US embassy headquarters. A single suicide attacker killed an Afghan family, including a human rights chief, with a bomb at a nearby supermarket three years ago, but security has been tightened since then at most places frequented by the Afghan elite or foreigners.

If the UN staff are confirmed as victims of the trio of attackers, it would make it the deadliest day for the United Nations in Afghanistan in nearly three years. In April 2011 a rioting mob overran a compound in northern Mazar-e-Sharif city, killing three UN workers and four Nepalese security guards. Eighteen months earlier in autumn 2009 five UN workers were killed in Kabul when gunmen burst into a guesthouse for the organisation's staff. Two Afghan security guards and an Afghan civilian were also killed in that assault, shortly before the second round of the last presidential election. The deaths had a far-reaching impact on how the United Nations and many other international organisations worked in the Afghan capital.

Friday's target, the Taverna restaurant, was a low-key but well-loved venue usually busy on a Friday, the Afghan weekend. It had guards with AK-47s and an air-lock entry system of steel gates, but those precautions would have been little match for a heavily-armed suicide squad.The area is packed with the homes of the Afghan elite, guesthouses for foreigners and offices of international organisations. Many streets are patrolled by police guards, but a well-prepared group could thread its way through back streets and avoid them.

An anxious man waiting outside said his father was missing and a friend had been taken to hospital with serious injuries.

"My dad called home this afternoon and said 'you guys have your dinner without me, because I am going to a restaurant with my friends'. We heard on the news about the explosion, and came to the site, and I saw my father's friend being carried out by the police," said Ajmal, who said his father, Mohammad Ali, worked for a telecoms firm.

"His friend was seriously wounded in his leg. My father has disappeared and we are very worried."