The first troops are due in Afghanistan by Saturday
The force, which he said would number between 3,000 to 5,000 in total, was designed to bring stability to Afghanistan and to ensure a period of order while a new administration takes over from the Taleban.
But Mr Hoon told MPs on Wednesday there were still many issues relating to the exact size and composition of the force to be finalised over the next few days.
He said that because there was still a need to conclude a military agreement with the Afghans, the main body of the force would not begin to deploy before 28 December and would take several weeks.
'Challenge'
He acknowledged the mission would be "a complex undertaking" which would produce its own difficult challenges.
The international force would be commanded by Britain's Major General John McColl.
Mr Hoon said it was anticipated that a UN Security Council resolution to authorise the deployment under Chapter 7 of the UN Charter would be agreed within the next few days.
After the initial three months, the UK would hand over lead nation status to one of its international partners, he said.
An advance party of 100 Royal Marine commandos are expected to arrive in Kabul on Saturday, the same day Afghanistan's interim government takes power.
They will not technically be part of the international security force, but will prepare the way for the force's arrival.
Diplomatic relations
Mr Hoon's announcement came shortly after Foreign Secretary Jack Straw revealed the UK was to renew diplomatic relations with Afghanistan after a break of 12 years.
Mr Straw said the Office of the British Representative in Kabul would take on formal embassy status on Saturday with a charge d'affaires, to coincide with the new interim administration.
Shadow defence secretary Bernard Jenkin said the Tories would fully support the deployment, but raised a number of concerns, including the dangers of "mission creep".
He also expressed concern that British troops could become a target for pro-Taleban elements, angered by the UK's role in the military campaign inside Afghanistan.
This followed earlier comments from Conservative leader Iain Duncan Smith, who said of the operation: "It's not that there shouldn't be peacekeeping, it's that British troops will find themselves straddling two stools and may themselves become surrogate targets."
The Tory leader also echoed the concerns expressed by former defence chief Lord Guthrie that UK military resources should not be overstretched.
British defence officials said the International Security Assistance Force would initially be confined to the Afghan capital Kabul, with the Northern Alliance agreeing to withdraw most of its forces from the city.
Mandate
The international force will be lightly armed and will not be equipped with armoured vehicles.
Mr Hoon said the British force would be properly equipped and have "robust rules of engagement".
It is thought it will have a United Nations chapter seven mandate, allowing it to take the initiative on the ground and not just act in self-defence.
Meanwhile, Prime Minister Tony Blair called British forces the "finest in the world" in a Christmas broadcast to UK troops.
Mr Blair told the British Forces Broadcasting Service: "My message to our armed forces is that Britain is hugely proud of what you have been doing, of what you are going to do."
In a separate interview for the BBC World Service, the prime minister said the UK would remain a staunch ally of the US as the coalition moved to shut down the al-Qaeda network completely in the world outside Afghanistan.