The plan is to deploy more troops to guarantee security
An estimated 1,500 British troops will lead the International Security Assistance Force (ISAF) - whose formation is expected to be formally approved at the United Nations on Thursday.
A contingent of 100 British Marines already in Afghanistan will be sent to Kabul by Saturday, when the country's interim government is to be sworn in. The main body of the force will not begin to deploy before 28 December.
Armed factions
But the force may not be operational until at least the middle of January because of its reliance upon Bagram airport 40km north of Kabul for both deployment and re-supply.
Mr Hoon admitted that the force - to be led by General John McColl - would face a "challenging, difficult and sometimes dangerous" task in Kabul, which is controlled by a number of armed factions.
BBC correspondent Daniel Sandford said: "It could go very well, but it only takes one warlord to take against it and it could become very difficult."
Mr Hoon spoke following a meeting in London of about 21 potential contributing nations to the force.
France has said it is willing to send as many as 800 soldiers to the multinational force, and Spain has offered a battalion of 700 combat troops. The Netherlands, Germany and Italy are also expected to be major European contributors.
UK defence officials told the PA news agency that failure to get agreement on the MTA would be a "red card" for the whole operation.
Some Afghan leaders have questioned the size of the force, which say they is too small, but Mr Karzai says he is satisfied with the numbers of troops.
Speaking at the end of a visit to Rome, where he met Afghanistan's former king, Mr Karzai outlined his views of the challenges ahead at a news conference.
On the international security force, Mr Karzai said, "If it takes force to keep peace, why not? The basic element is to keep peace."
But he has already stressed that such a force should be a purely temporary measure, only needed until the new Afghan Government was able to put in place its own force.
Regarding former members of the Taleban, Mr Karzai said those who had no criminal record would be freed. Those who had would be tried according to Islamic Sharia law.
Mr Karzai said if Osama Bin Laden were caught he would be handed over to international justice.
Mr Karzai will be sworn in on Saturday as the head of the new interim administration in Kabul, which he will lead for six months, paving the way for a two-year transitional government.