Jaap de Hoop Scheffer said there were high stakes for international security
Nato head Jaap de Hoop Scheffer has warned that walking away from the alliance's mission in Afghanistan would have a "devastating" effect.
Speaking in London, the Nato secretary-general said failure would give free run to al-Qaeda.
His comments come as Afghanistan suffers a spike in violence ahead of elections on 20 August.
More foreign troops have been killed in July than in any other month since the US-led invasion in 2001.
In a speech at a think tank, Mr de Hoop Scheffer said Nato allies could not afford to abandon their campaign.
"If we were to walk away, Afghanistan would fall to the Taliban, with devastating effect for the people there - women in particular," he said.
He also said any such move would have an impact on the wider region.
"Pakistan would suffer the consequences, with all that that implies for international security," he said.
"Central Asia would see extremism spread. Al-Qaeda would have a free run again, and their terrorist ambitions are global."
He said Nato members had to realise that the mission was "essential" to their security.
"As much as we may long for the near-perfect security of Cold War deterrence, we must accept that security today requires engagement in far away places - engagement that is dangerous, expensive, open ended, and with no guarantee of success."
Earlier on Monday, the Nato-led force in Afghanistan announced the deaths of four US soldiers in the east of the country.
The deaths bring the number of Nato soldiers killed in July to 56.
Eighteen of those are from the UK, where the rising toll has sparked debate over the country's participation in the Nato-led mission.
Mr de Hoop Scheffer, who met UK Prime Minister Gordon Brown earlier on Monday, acknowledged the sacrifices being made by soldiers from the UK and other allies.
He also called for what he called more equitable "burden sharing" between members of the Nato alliance.