The US president called on member states to scrap the numerous caveats that prevent their forces from fully engaging in the Afghan mission.
He said: "Like Estonia, member nations must accept difficult assignments if we expect to be successful."
Jaap de Hoop Scheffer, the Nato secretary general, has insisted that the alliance can succeed in Afghanistan and called Nato's operation "mission possible".
Scheffer said it was unacceptable that allied forces in southern Afghanistan, the main battleground with resurgent Taliban fighters, were 20 per cent below the required strength.
he said: "While we have to be frank about the risks, we also have to avoid over-dramatising the difficulties."
He was speaking a day after a suicide bomber killed two Canadian soldiers in the latest attack on a Nato convoy in southern Afghanistan.
|Taliban fighters have intensified their|
campaign against Nato forces
The violence promptied Canada's foreign minister to warn public support could turn against the mission if allies did not come to Ottawa's aid.
NATO commanders are also frustrated by around 50 caveats, which range from geographical restrictions - the biggest problem - to the refusal to fight at night or in winter conditions.
The British prime minister, Tony Blair, has also urged Nato to do more in Afghanistan.
He told a news conference held with Anders Fogh Rasmussen, the Danish prime minister: "NATO's credibility is at stake here. If we don't succeed in Afghanistan the whole of the world will be less secure."
Jacques Chirac, the French president, has called for a "contact group" to be formed of countries neighbouring Afghanistan and those involved in the Nato mission.
Chirac, writing in Britain's Guardian newspaper, said such a group was "necessary to give our forces the means to succeed in their mission in support of the Afghan authorities, and refocus the alliance on military operations".
Chirac wrote: "The Europeans have relied on their American allies for too long ... They have to shoulder their share of the burden by making a national defence effort commensurate with their ambitions for Nato and also for the EU."
We a need a body like the ... contact group in Kosovo that brings the key international actors together on a regular basis and coordinates overall strategy."
Scheffer voiced support for Chirac's plan, saying: "
Meanwhile, the Financial Times newspaper reported that Chirac will announce that the restrictions that prevent French troops in Afghanistan from entering the fighting in the south are to be loosened.
According to an unnamed senior French official, Chirac will say that in future, French troops could be deployed around the country when they are needed to help their Nato allies.
In early September, US General James Jones, Nato's military commander, called for 2,500 extra military personnel for southern Afghanistan - about 1,000 combat troops supported by 1,500 logistical and other staff - but they have not yet been provided.