Two United Nations doctors working on a polio vaccination campaign for children were killed by a suicide bomber yesterday in the first significant Taleban attack against the organisation in Afghanistan.

The men, both Afghan nationals, died with a driver when the bomber rammed their UN vehicle in the market town of Spin Boldak, close to the border with Pakistan in the southern province of Kandahar.

The attack was the first to single out marked UN vehicles and compounded an increasingly jittery atmosphere among the expatriate aid and diplomatic community in Kabul. The convoy of two armoured Land Cruisers and three police vehicles carried UN and World Health Organisation markings, according to witnesses. Afghan officials said that 15 people were wounded in the attack.

A statement issued by the Taleban on their website claimed responsibility for the attack: “This morning Taleban conducted a martyrdom attack by a hero, Abdul Salaam, in the centre of Spin Boldak against a convoy of Unama [UN Assistance Mission Afghanistan]. Two Land Cruisers of the convoy were targeted and eight highranking foreign officials were killed.”

In a statement the UN special representative in Kabul said: “This attack was on innocent civilians working only for the people of Afghanistan, and is beyond comprehension.”

Adrian Edwards, the chief UN spokesman in Kabul, told The Times: “We are looking into this with a view to assessing if this was a target of opportunity or if this was an explicitly planned attack. We rely on the acceptance of the community. This looks like it marks a change in the situation.”

In recent weeks the UN has put out a number of warnings to its staff after explicit threats to the organisation were picked up by intelligence agencies. Western officials rejected as alarmist any suggestion that the UN had any imminent plan to withdraw staff. However, the UN maintained plans to shift non-essential staff to neighbouring countries if security deteriorated significantly.

After a bomb in Iraq in 2003, which killed 22 of its staff, the UN left and the country quickly fell into chaos.

Last month three women aid workers, one of them a British citizen, from the aid agency International Rescue Committee were shot dead on a road south of Kabul. Western intelligence agencies have monitored an increasing number of attacks against non-military targets, apparently designed to foment an atmosphere of fear and instability, particularly around Kabul.

Aid agencies in Kabul reported that their security measures were under review, but none planned to leave.

“We realise we have to be proactive on security rather than reactive,” said Jamie Terzi, the assistant country director for CARE International, an aid agency that has worked in Afghanistan since 1961. “The situation has changed. It is a different type of Taleban from that of the past. The old Taleban would never have killed women, as they did last month.”

On Saturday the provincial governor of Logar province, south of Kabul, and two bodyguards were killed. Militants also killed seven police officers, including a district police chief, in the province of Ghazni. More than 900 Afghan police officers were killed in 2007. Yesterday a provincial official and his bodyguards were killed in Zabul and a translator working with Western forces was killed in Khost.

US forces carried out raids at the weekend that killed a number of alleged militants linked to the insurgent network of Siraj Haqqani, a militant leader described by US commanders as one of a new breed of more radical and ruthless Taleban with close links to al-Qaeda.

On Saturday a British soldier from the 2nd Battalion, The Parachute Regiment, was killed in Helmand. He was the third to die in the past week.

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