Tony Blair praised the role of British forces
Promising not to "walk away" from the Afghan people, Mr Blair said food aid must urgently be stepped up in the wake of the advances by the opposition Northern Alliance forces.
But he also emphasised the objectives of the military campaign, including bringing Osama Bin Laden and his associates to justice, had not yet been achieved.
"The military campaign is not over until the objectives are fully secured," he told reporters.
BBC Political Editor Andrew Marr said talk in Downing Street had once again turned to a possible role for Afghanistan's exiled king in any transitional government.
He added that as well as significant military advances, there were now large scale defections from the Taleban.
This raised hopes that defecting Taleban fighters would bring with them information crucial to the military alliance and the hunt for those behind the US terror attacks.
Tribute to British forces
The speed of the Taleban's retreat was a tribute to the professionalism of the coalition forces, said Mr Blair, who gave special praise to British troops operating in Afghanistan.
Mr Blair earlier held an hour-long meeting at Downing Street with senior military and other advisers after Northern Alliance fighters moved into Kabul.
Downing Street says the Taleban's departure from Kabul does not seem to be a tactical defeat and Mr Blair reinforced that message.
"The military strategy aimed at defeating the Taleban is clearly succeeding - they are in disarray and retreat," he said.
"However, our job is not yet done by any means.
"We need urgently to put in place the next political and humanitarian moves that the changing military situation now permits."
Cautious tone
The government's cautious tone was adopted as it published its legislative response to the 11 September terror attacks, involving a range of new powers for security services and a shake-up of existing laws.
But the political situation on the ground in Afghanistan now appears to be the most pressing issue for Western governments, which have appealed for the Northern Alliance to hold back from entering Kabul until a broad-based government can be formed.
Talks about the make-up of a successor government to the Taleban are understood to be underway at the UN, whose secretary general, Kofi Annan, spoke to Mr Blair on Tuesday.
Mr Blair said: "I believe that we can make real progress towards the filling of the current power vacuum there but we need a UN presence there as soon as possible."
That required making quick progress in assembling the various different elements needed for a broad-based successor regime.
Home Secretary, David Blunkett, said the recent behaviour of the opposition Northern Alliance showed a "new discipline and understanding" from their forces.
And he argued the latest advances created new opportunities for the humanitarian aid effort.
Criticisms remain
Critics of the military campaign have rejected claims the latest developments prove the US-led bombing in Afghanistan is working.
Labour MP Paul Marsden, who has just returned from Afghanistan, said: "I do not think anyone could possibly say the bombing has done the trick.
"The cost to hundreds of civilians has been terrible deaths and injuries."
He said the Northern Alliance had inflicted gruesome punishments and looted humanitarian supplies.