Thousands of troops are still on standby
About 100 troops, believed to be members of the Special Boat Service, are there to carry out a fact-finding mission.
Defence officials say the troops will be checking out the airport's facilities in preparation for humanitarian operations.
It is believed they will also be assessing whether conditions are right for the future deployment of more ground troops.
Foreign Secretary Jack Straw announced that Stephen Evans, 51, would be helping opposition forces to set up a broad-based government.
"Taking prompt action is an important way of helping to secure the advances that have already been made during the course of the week," Mr Straw said.
News of the British deployment came as Tony Blair warned the campaign against terrorism was far from over despite his claims that the Taleban were near "collapse".
Troops on standby
A senior British defence source said on Thursday night: "I can confirm that some of our forces are on the ground in northern Afghanistan, having arrived at Bagram airport at 1200GMT.
"They are to carry out technical tasks in connection with future humanitarian tasks."
Last Sunday, Defence Secretary Geoff Hoon confirmed that British troops were already on the ground in Afghanistan.
He did not go into details, but it was thought that the UK forces involved were members of the SAS, and that they had been in the country for several weeks.
It is believed the troops which landed on Thursday will secure Bagram airport and then clear a pathway into the Afghan capital for any future aid missions.
Bagram aiport, situated about 10 miles north-east of Kabul, is considered to be of key strategic importance.
Thousands of UK troops will remain on 48-hour standby to go to Afghanistan, while the advance party of Royal Marines assesses the situation on the ground, MoD sources said.
Frontline duties
The units are said to include the whole of 2nd Battalion The Parachute Regiment, the entire 45 Commando of the Royal Marines, and elements from 16 Air Assault Brigade and 3 Commando Brigade.
Mr Blair has said they could be used in future frontline offensives against the Taleban, although protection of humanitarian supplies and "stabilising" work would be the priority.
An MoD spokesman said that although there were a large number of British troops on standby, "there is no assumption yet that they are definitely going to be deployed".
Earlier on Thursday, Mr Blair said the situation in Afghanistan was "infinitely better" than a few days ago, despite unconfirmed reports of Northern Alliance atrocities.
At a news conference in central London the prime minister said that with Taleban resistance largely broken the chances of ensuring a stable future government for the war-torn country had improved dramatically.
Mr Evans, a former lieutenant in the first Royal Tank Regiment, said the security situation had been considered carefully.
"We are confident that at the moment we can deploy safely to the Kabul area."
Britain's previous ambassador to Afghanistan left Kabul when Soviet forces invaded in 1979.
Earlier on Thursday, Mr Straw pronounced himself "relatively optimistic" over the prospect for peace in Afghanistan - provided events move swiftly.