UK forces will remain in Afghanistan beyond 2009, Defence Secretary Des Browne has told a committee of MPs.
British soldiers have been deployed to Helmand for a year
He said February's announcement of a troops boost, with forces committed to 2009, was "for planning purposes only".
"My own view is that we will have to...stay with the Afghans for beyond 2009 but exactly how long and in what way it is too early to tell," he said.
Mr Browne told the defence committee it could "take decades" for Afghanistan "to stand on their own two feet".
British troop numbers are due to increase by 1,400 to 7,700, with most of the new contingent being deployed this summer to the volatile province of Helmand, where UK forces have been fighting the Taleban.
'Decades' of support
Mr Browne said the timeframe for British troop involvement was uncertain, but reminded the committee that in 2006 the international community had agreed with the Afghan government to a further five years of support.
The British government also made a bilateral agreement with Afghanistan for 10 years of support.
"I realise that that doesn't mean that we...committed to a military presence for either of those two periods," Mr Browne said on Tuesday.
"I believe that this country (Afghanistan), having been 30 years in conflict or more, will take decades to get to the stage where they will be able to stand on their own two feet and that the international community will have to support them for a considerable period of time."
He said UK troops had been in Helmand for only one year and were still learning about the challenges there.
The emphasis for UK troops in Helmand is counter-narcotics, as the province is the largest single source of opium in Afghanistan.
Future support would depend on building up the Afghan national army and its police force, Mr Browne added.
But he said that military and financial support from the international community would be key.
British troops operate under the Nato-led Isaf force, a United Nations-mandated organisation covering about 31,000 troops.
They are backed by about 30,000 Afghan troops and a similar number of Afghan policemen.