Two British soldiers were killed in clashes in Afghanistan on Saturday, the Ministry of Defence has said.
British troops are experiencing heavy fighting in the south
The two men, both from 29 Commando Regiment Royal Artillery, were killed in a Taleban rocket attack in Helmand in the south of the country.
Brig Jerry Thomas, Commander of Afghanistan's UK forces, said losing two men was a "deeply saddening blow".
In a separate incident, 16 civilians died in what was described by US forces in Afghanistan as a "complex ambush".
The British soldiers were taking part in an operation in the Sangin area of Helmand province as part of the Nato-led International Security Assistance Force when they were killed.
Brig Thomas said: "The loss of two of our men in one incident is a deeply saddening blow.
"It will be particularly badly felt by all those back home and I would like to sincerely convey my thoughts and condolences to the men's families and friends at this very difficult time."
The families of the two men had been informed, he added.
The BBC's Alastair Leithead said British troops in the south of the country had been clashing daily with Taleban forces and had experienced heavy fighting over the last few days.
A large number of insurgents were crossing the Pakistan border to attack Nato forces, our correspondent said.
The two deaths come just days after Defence Secretary Des Browne announced 1,400 extra troops would be sent to the country, increasing the number of UK servicemen and women to 7,700.
The number of UK troops killed in combat operations in Afghanistan since 2001 is now 50. Of these, 22 died from accidents, illness, or non-combat injuries, according to the MoD.
In a separate incident, a suicide bomber targeted a US convoy, sparking a fire fight on the road from the eastern city of Jalalabad to Pakistan.
A total of 16 civilians were killed in what was described as a "complex ambush" by US forces.
The US military said a minibus containing explosives was driven at the convoy, injuring one soldier.
Twenty foreign soldiers have been killed in Afghanistan this year and Nato fears that Taleban violence will escalate in several areas of the country as the winter snows thaw.
Last Tuesday at least nine people were killed in a suicide bombing at the main US base in Afghanistan, Bagram near Kabul, during a visit by US Vice-President Dick Cheney.