Mr Karzai insisted his forces are ready to take on defence of the country.
A statement from the president's office said the partial pull-out is an "appropriate" move as Nato forces hand over the war against the Taliban to the Afghan military.
Prime Minister David Cameron announced on Wednesday that about 3,800 British troops would be withdrawn by the end of 2013, leaving some 5,000 into 2014.
The majority of Nato forces, including those of the United States, will depart by the end of 2014.
Meanwhile, in France, Afghan officials are meeting Taliban rebels and envoys from another Islamist militant group near Paris, looking beyond Afghanistan's uprising to a future long after international forces have returned home.
French hosts say the secretive, rare two-day meeting among rival Afghans in Chantilly - known partly for its equine training grounds - is not expected to involve any horse-trading towards a possible peace and reconciliation deal.
About 20 Afghans from President Hamid Karzai's government, the Taliban, as well as the political opposition and the Islamist Hezb-e-Islami militant group will be merely trying to foster a conversation after 11 years of war.
The meeting comes as France is pulling its last combat troops from Afghanistan - well ahead of Nato's withdrawal timetable.