Speaking in Edinburgh at a Nato parliamentary assembly meeting, Anders Fogh Rasmussen, said: "In a few weeks, I expect we will decide, in Nato, on the approach, and troop levels needed, to take our mission forward."
Barack Obama is expected to make a long-awaited declaration on US troop levels and strategy in the next few days. But Rasmussen pre-empted the president by predicting the alliance as a whole would pursue a broad counter-insurgency approach, requiring many more soldiers, rather than the narrower focus on counter-terrorism – such as targeting suspected jihadist leaders – advocated by the US vice-president, Joe Biden.
"I'm confident it will be a counter-insurgency approach, with substantially more forces," Rasmussen said, and promised there would soon be "new momentum" behind Nato's beleaguered Afghan mission.
His announcement follows a meeting of the North Atlantic Council last week, in which the alliance's member states broadly endorsed a strategy proposed by the US commander in Afghanistan, General Stanley McChrystal, focused on protecting the civilian population and bolstering the Afghan government.
Gordon Brown said today that 10 Nato member states would be prepared to send extra troops. He has pledged 500 more British soldiers, bringing the UK force to 9,500. Brown said the alliance could send as many as 5,000 alongside the US deployment announced by Obama.
Slovakia said it would raise its troop contribution from 250 to 500 today. The rest of the new forces will come from Nato members such as Turkey and Germany, which is expected to announce the size of its deployment in the new year. Others will come from Nato's partners in the International Security Assistance Force. South Korea will send hundreds of troops to create a new "provincial reconstruction team", probably in Parwan province, north of Kabul. Mongolia is also expected to send a significant contingent.
In another speech to the Nato parliament today, David Miliband called for a new emphasis on building up local government and traditional councils. The foreign secretary noted that the average Afghan government budget for each province was less than $1m (£600,000).
He said there should be more resources pumped into local government which should in turn help recreate representative shuras – traditional councils.
"They can provide a forum for a political debate, and, under carefully controlled conditions, provide the re-entry mechanisms for insurgents seeking reintegration," he said.
Miliband restated British government support for a policy of "flipping" Taliban footsoldiers by offering better livelihoods, followed by overtures to more senior militants. "Once reintegration gains momentum, and the insurgency is stating to fray or crumble, we will need to support President [Hamid] Karzai in reaching out to those high-level commanders that can be persuaded to renounce al-Qaida and pursue their goals peacefully within the constitutional framework," he said.
Western officials predict that one of the most difficult parts of the strategy will be working with the re-elected Karzai government, which Brown has described as "a byword for corruption".
Afghanistan was today declared the second most corrupt country in the world, behind Somalia. The ranking, published by a watchdog group, Transparency International, showed that Afghanistan had slipped three places in its Corruption Perceptions Index over the past year, despite the huge investment and close attention of Nato and international donors. Iraq was fifth worst in the list of 180 countries.
Under intense pressure from Washington, Karzai has announced the creation of an anti-corruption commission, built on the model of the FBI. It is unclear whether there will be any role in the new institution for foreign oversight. British officials believe it should be an entirely Afghan organisation. There is a debate under way between Nato allies and the Karzai government over the creation of new international co-ordinator, or high representative, in Kabul to help ensure that reconstruction money is spent properly.
Miliband will represent Britain at Karzai's inauguration on Thursday at a ceremony that will be closed to journalists.
"Due to capacity and security reasons we cannot invite the media into the palace," a palace spokesman, Siamak Herawi said yesterday.
A survey published today by Oxfam found one in five Afghans had been tortured and one in 10 imprisoned at least once since the 1979 Soviet invasion. Only 1% of those interviewed said they had received any form of compensation for their treatment. Asked about the causes of the conflict 70% of Afghans named poverty and unemployment.