Nato's top civilian official in Afghanistan warns of further deaths in 'very tough year' for British and other foreign troops

British and other foreign troops deployed in Afghanistan face a "very tough" time ahead and can expect to be engaged in a combat role for three or four more years, Nato's most senior civilian official in the country said today.

Mark Sedwill, a former UK ambassador to Afghanistan, warned of further troop deaths in the region, saying: "We cannot allow judgment of success to be the absence of casualties."

Speaking on the margins of a Royal United Services Institute conference in London, Sedwill laid out crucial steps towards ending the conflict, to be taken over the next few months.

A "critical test" would be the imminent operation in Kandahar designed to improve security and governance in the Taliban's heartland, he said.

That operation was not primarily a military one, Nato insists. However, as more troops are engaged in counter-insurgency operations in southern Afghanistan, it was inevitable there would be more casualties, Sedwill said.

He said the "bumper sticker" message for the next British government had to be that "a stable Afghanistan is a safer world".

The stability of Pakistan was also at stake, he said, as was that of other places vulnerable to al-Qaida-inspired extremists. The message to these countries had to be: "We stand by you."

There was a gap between public opinion, with polls reflecting growing sceptism about Nato-led operations in Afghanistan, and the political leadership in Nato countries, Sedwill said.

Casualties among British and other Nato troops have increased significantly over recent years. More than 100 UK troops were killed and over 150 seriously wounded in Afghanistan last year, more than double the numbers for 2008.

Thirty-three have been killed so far this year. "Expect 2010 to be again a very tough year", Sedwill said.

British and other Nato troops could be expected to be engaged in combat roles for "another three or four years", he said. Thereafter, they could be expected to remain in Afghanistan, training and mentoring local forces, for a further 10 to 15 years.

Barack Obama has indicated that the number of US troops in Afghanistan – still rising – will begin to fall from July 2011. However, Sedwill suggested that the number of UK forces would not start to shrink at the same time as they would be needed to train Afghan troops. Senior Nato officials say they are seriously concerned about the shortage of experienced troops to train local forces.

The aim was to announce the first handover to local forces of responsibility for security in a number of Afghan provinces at the Nato summit in Lisbon in November, Sedwill said. But it was important, he added, not to expect an end of violence to be a precondition for a handover. The questions were what was an an acceptable level of violence in Afghanistan and, as important, the degree to which Afghans felt that their security and institutions of government were improving.

As Nato and Afghan forces prepare for their Kandahar operation, the Afghan president, Hamid Karzai, is due to visit Washington on 10 May. Later that month he is due to preside over a peace jirga of tribal and community leaders, designed to boost a much-discussed reconciliation and reintegration process.

In order to benefit from the process, leaders had to accept the existing Afghan constitution and renounce violence as a political weapon, said Sedwill. Although the emphasis now was on the political, governmental, and economic, development of Afghanistan, "military pressure" still played a key role. That, he said, was important in what he called "draining the fuel out of the insurgency".

Three-quarters of the insurgents were "allied with the Taliban rather than fighting for it", Sedwill said, implying that the large majority were not ideologically motivated. He described corruption as a "symptom of abuse of power", an issue between Afghans rather than one between Afghans and the international community.