Nato assumed responsibility for security across the whole of Afghanistan today when it took command in the east of the country from US-led coalition forces.
Its International Security Assistance Force (Isaf) already commanded forces in the north, west and south, as well as in the capital, Kabul.
Today it took command of about 12,000 US troops in the east.
The US troop transfer had been expected later in the year, but alliance officials said battles with resurgent guerrillas in the south showed the urgent need to pool British, Dutch and Canadian troops under Nato with the US forces.
With the inclusion of the US troops in the east, the Nato force in Afghanistan will number about 31,000 troops. About 8,000 US troops remain in a separate force, concentrating on counter-terrorist operations and training Afghan forces.