Reid arrived in Kabul Sunday and held talks with his Afghan counterpart, Abdul Rahim Wardak on the deployment of some 4,000 British troops to southern Helmand province.
"Most of the discussion which took place today was for better coordination of our forces -- and the British forces in Helmand area and with all NATO forces," Wardak said.
Reid told a press conference: "There are two missions in Afghanistan, one is the operation Enduring Freedom which is the anti-terrorism mission controlled and led by Americans.
"There is a second mission ... Britain is playing a major part of it, which is the reconstruction of Afghanistan by multinational forces," he said.
British troops were going to southern Afghanistan to help Afghan people reconstruct their economy and build democracy and would be happy to leave in three years without firing one bullet, he said.
"The counter terrorist mission of the Americans would not be happy if they left in three years time without firing a shot because they are there to go after and chase and kill and destroy the terrorists," he added.
"That is the essential difference between the two missions."
Reid said counter terrorism forces must continue the offensive against Al-Qaeda and the Taliban to ensure they did not return to power.
"The greatest danger of all for the people of Afghanistan and the people of the United Kingdom would be if Afghanistan ever again came under the rule of a Taliban regime prepared to protect Al-Qaeda and terrorists groups."
Reid was due to meet Afghan president Hamid Karzai and British troops stationed in Afghanistan.
Britain initially stationed 1,100 soldiers in Afghanistan. It has started sending additional troops to Helmand, site of a string of suicide and roadside bomb attacks.
Some 4,000 British troops would be deployed to the region along with a team of engineers but the number would eventually fall to 3,500, he said, adding the province was the most dangerous in the country.
Taliban militants have been attacking Western and Afghan targets since their hardline regime was ousted in late 2001 by US-led forces for refusing to hand over Al-Qaeda chief Osama bin Laden after the September 11, 2001 attacks.