Speaking to a meeting of defence ministers in France, Scheffer said the International Security Assistance Force (ISAF) providing security in Afghanistan now had the resources to make such an expansion possible.
Nato took command from the American-led ISAF force in 2003 and the US repeatedly called on its European Nato allies to commit more troops in Afghanistan and in Iraq
It currently comprises some 8300 soldiers from more than 30 countries, deployed in the Afghan capital Kabul and the north in the form of provincial reconstruction teams (PRTs).
Last year NATO's political leaders agreed to extend into the more remote west of the country. But military chiefs had struggled to drum up the necessary contributions on the ground.
The alliance is reluctant to reveal details, but officials have
indicated that Italy, Spain and new member Lithuania are among NATO countries willing to take over PRTs in the west and a forward operating base in the city of Herat.
US welcomes decision
NATO Secretary-General Scheffer
had lobbied for more troops
For the United States, the NATO decision is welcome news, as it seeks to ease the pressure on its forces committed both in
Afghanistan and Iraq, its two key frontlines in the "war on
terrorism" triggered by the September 11 attacks.
The commitment to expand will only add a further 500 troops for the time being with more promised if needed.
NATO insists the addition of four so-called "provincial reconstruction teams" in the west will mean it can give much greater help to the separate US-led force in stabilising the country before parliamentary elections due by June.
Under current plans, Spain is expected to create a unit in Qalah-i-Nau, northeast of Herat, and Lithuania to go to the town of Chaghcharan. Italy will support a US unit in Herat.
In a bid to show unity within an alliance split over the Iraq war, NATO has also called on all members to promise help in training Iraqi forces by the time of a 22 Febuary summit at its Brussels headquarters with US President George Bush.
NATO agreed in June to form a mission in Iraq to train 1,000 Iraqi officers a year but has not yet raised the staff needed for full training to start. France, Germany and other opponents of the US invasion have refused to set foot in Iraq.