At least six people including a British embassy guard were killed on Thursday when the Taliban launched a devastating suicide attack on a British diplomatic convoy near the Afghan capital Kabul, in a day of mayhem that also saw gunfire in the city.

The convoy was travelling on the road between Jalalabad and Kabul – about three miles east of the heavily fortified British embassy – when a suicide bomber struck. The bomber’s Toyota Corolla vehicle exploded – hurling the armoured British embassy SUV across the road and blowing off its roof completely. Smouldering debris was flung across a packed area including a mosque.

Later on Thursday there was a second explosion in Kabul’s Wazir Akbar Khan district, where the British embassy and other foreign missions are located. According to Afghan police, three Taliban fighters attacked a foreign guesthouse at 7.30pm. One fighter blew himself up while the other two staged a shoot-out with Nepalese security guards. Both attackers were eventually killed, police said. The compound belonged to International Relief and Development, a development agency.

The UK foreign secretary, Phillip Hammond, said that two British embassy workers died in the morning’s convoy attack. One was a British civilian security team member and the other an Afghan national working at the embassy. A second British member of the security team was injured, Hammond said. The private security firm G4S said both the dead security guard and his injured colleague were its employees.

Hammond described the bombing as an outrage and an “appalling attack on innocent civilians”.

“The families of the victims have been informed and my thoughts are with them,” he said. The dead Briton has not yet been named.

Afghanistan’s interior ministry said four Afghans were killed in the attack and another 33 civilians were wounded.

Downing Street defended the UK’s decision to withdraw troops from Afghanistan and also condemned the suicide attack as “appalling”.

The prime minister’s official spokesman said David Cameron’s view had not changed that it was right for the UK to pull out of the country next month.

Cameron’s spokesman said: “What I would say on his behalf is to condemn the appalling attack and to associate the prime minister entirely with the sadness about the fact British embassy staff have been involved in that attack. We will be working very closely with the Afghan authorities to establish all the facts and details related to the incident.”

He said the UK had accomplished its objective in Afghanistan to free the country of training camps that were a haven for terrorists.

“The right way going forward is to have a stable Afghan government that represents its people and I believe you see that in the recent round of Afghanistan presidential elections,” the spokesman said.

The explosion could be heard across Kabul and a plume of smoke rose above the site of the blast. The Jalalabad-Kabul road is a key highway connecting several foreign compounds and military bases outside the capital with downtown Kabul. The road has been the scene of numerous recent insurgent attacks. On 19 November, a suicide bomber blew up his vehicle outside a walled UN compound nearby, while last Monday two American soldiers were killed further down the road towards Jalalabad by a remotely detonated explosive.

Immediately after the blast, an AFP reporter saw one badly shaken foreign passenger talking to Afghan police, asking after his wounded colleagues.

A policeman at the scene said one Afghan in the British vehicle had suffered a serious arm injury, and that passengers covered in blood had been taken to hospital.

“One British national was killed in today’s attack,” police spokesman Hashmat Stanakzai said in an emailed press statement.

The British ambassador in Kabul, Sir Richard Stagg, said in a statement: “A British embassy vehicle was attacked by a suicide bomber on the Jalalabad road. It contained one Afghan member of our staff and a two-man close protection Team. One of the team survived.

“My thoughts - and those of others in the embassy - are with our injured colleague; and with the families and friends of those killed. They can be proud of the contribution which the three individuals have made to the work of the embassy.”

The embassy already uses some of the strictest security protocols in Kabul. There is very limited travel, only for events considered essential, with the embassy quite often on total lockdown with no movement in and out. When staff do travel they move in armoured vehicles, with G4S drivers and security, and with always more than one vehicle.

The bombing happened at around 10.30am local time. According to one report, a translator was among the dead.

According to the local Pajhwok news agency, the Taliban have claimed responsibility for the attack, the latest in a series in recent days that have killed dozens and wounded hundreds.

The attack on a high-profile British target appears to send a deliberate political message. It comes a week before a major conference on Afghanistan on 4 December in London, co-hosted by the UK and Afghan governments. According to Downing Street, the conference to be attended by international donors will “provide a platform for the government of Afghanistan to set out its vision for reform”. It will also give the international community an opportunity to demonstrate its continuing commitment to Afghanistan, as the majority of foreign combat troops withdraw after 13 years of war against the Islamist Taliban and their allies. A smaller support mission will stay behind.

At the scene of the blast on Thursday, twisted chunks of metal had been flung over a large area covered in pools of blood. Children working as scrap collectors gathered the remains of charred vehicles. It appears two British diplomatic SUVS were involved with the first taken away. A second lay across the street, having been flung 40 metres away by the impact of the bombing.

Mohammad Abaseen, 24, a former interpreter for the British forces in Helmand, said the bombing occurred in front of a busy neighbourhood mosque. “Men were sitting under the sun, watching cars pass by. Many of them were killed or were injured in the blast,” he told the Guardian.

Abaseen said that one of those killed, Haji Azam, a local cobbler, had lost two of his sons in previous blasts. Azam died in hospital after suffering severe headwounds.

The threat level in Kabul has been higher than usual over the past few weeks. Intelligence officials have warned against an increased kidnapping threat against foreigners in Kabul and in some eastern provinces.

The attack underlines the challenge facing the country’s new president, former World Bank technocrat Ashraf Ghani, in tackling an insurgency that has flared up over the past months.

The Taliban have long sought to use violence in Kabul itself to undermine confidence in the Afghan government and its foreign supporters, as well as to sap backing for continued involvement in the country in the west.

While Afghanistan’s military and police remain in control of all 34 provincial capitals, violence has risen in the last year and the rate of casualties suffered by local security forces has been described by the US military as unsustainable.

As foreign troops withdraw, Taliban militants have intensified assaults on government troops, particularly in provinces in the east and south.

About 4,600 members of the Afghan security forces have been killed already in 2014, more than 6% higher than the same period of 2013.

This year has also been one of the bloodiest for Afghan civilians, according to the United Nations, which recorded nearly 5,000 deaths and injuries of civilians in the first half of the year.

Barack Obama recently issued new orders to increase slightly the number of American troops who will stay in Afghanistan and to allow them a more significant combat role than previously envisaged.