British troops could end their combat role in Afghanistan even sooner than the five years the government has suggested, the UK's top diplomat in the country said today.

Political developments could accelerate the process, leading to a reduction in fighting and to Nato forces ending their combat in a "three- to five-year timescale", said Sir William Patey, Britain's ambassador to Kabul. Talks leading to a political settlement should get off the ground sooner rather than later, he added, referring to contacts with Taliban elements.

He was speaking at a meeting in London organised by the International Institute for Strategic Studies, at which a Foreign Office minister, Alistair Burt, emphasised the government's commitment to ending the combat role of British troops in Helmand at the latest by 2015, the date Britain's next general election is due.

Burt was speaking as the Ministry of Defence announced the death of another member of Britain's armed forces in Sangin, which is regarded as the most dangerous place for foreign forces in Afghanistan.

The soldier, from 5th Regiment Royal Artillery, died after an explosion while he was on a foot patrol, the MoD said.

Nearly a third of the 313 British deaths in Afghanistan since 2001 occurred in Sangin, where 1,000 Royal Marines of 40 Commando and supporting units are based.

The defence secretary, Liam Fox, told MPs on Wednesday that British troops would pull out of Sangin in the autumn and be replaced by US marines. The handover will leave the British military effort concentrated in a smaller area of central Helmand.

At today's meeting, which was attended by Homayoun Tandar, the Afghan ambassador to Britain, both Burt and Patey stressed that though British soldiers would no longer be fighting in Afghanistan in five years' time at most, that would not mean the end of the UK's role in the country.

"David Cameron has made it clear he did not expect to have combat troops [in Afghanistan] after 2015 ... We made a judgment," Burt said. However, he added: "The withdrawal of combat troops did not mean the withdrawal of the UK from Afghanistan."

Patey spoke of a "long-term strategic partnership" between Britain and Afghanistan. He said Afghanistan had been neglected and it was only in the last year that resources "that can deliver" had been put in the country.

Both Burt and Patey emphasised the significance of the international conference due to he held in Kabul on 20 July, the need for a political settlement and the importance of reconciliation. Burt pointed to Cameron's visit to Delhi at the end of the month. It was important to "make very clear to India and Pakistan their engagement is absolutely crucial", he said.

However, preparing public opinion for more British deaths in Afghanistan, he said: "A very tough year is coming up. It will be difficult over the next few years."

Patey said drugs – opium poppy and heroin production in Afghanistan – were the "one issue" which could be the catalyst leading to the engagement of regional powers, including Russia and Iran, in a future political settlement in Afghanistan. Russia was also comcerned about Islamic extremism, he added.