Three NATO soldiers were killed when their patrol was struck by a bomb in southern Afghanistan and then ambushed by insurgents, the alliance force said on Friday.
Thursday's killings exacerbate a bloody start to the month for international troops helping quell an extremist insurgency, with 15 others killed so far, according to icasualties.org, which compiles a toll.
"The patrol responded to the attack but three service members died in the engagement," the International Security Assistance Force said in a statement.
ISAF did not give the nationalities of the dead, leaving this to their home countries. Most soldiers in the south, an insurgent hotbed, are American, British or Canadian.
Thousands of US Marines poured into the south earlier this year to try to secure militant strongholds ahead of August 20 presidential and provincial council elections.
British troops have been under pressure in southern Helmand province, hub of a drugs trade that helps fund the insurgency.
Last month was the deadliest for foreign troops in Afghanistan since the 2001 US-led invasion forced the extremist Taliban regime from power for sheltering Al-Qaeda following the September 11 attacks.
Most of the 76 deaths were in hostile attacks, the independent icasualties said.
There are more than 100,000 international soldiers in Afghanistan, including 62,000 Americans, according to Pentagon figures.