A Taliban attack has killed three American soldiers and a military dog in southern Afghanistan, officials said, in the latest sign of the insurgents' capability as the coalition winds down operations.
NATO's International Security Assistance Force will end its combat mission later this year, leaving the Afghan police and army to take on full responsibility for battling the resilient Taliban insurgency.
"Three ISAF service members and one ISAF canine died following an improvised explosive device attack in southern Afghanistan today," the NATO force said in a statement.
US defence officials confirmed they were Americans.
Military dogs are often used to sniff for explosives, though they have proved unpopular with Afghan civilians as they are seen as unclean by many Muslims.
The attack occurred in the volatile Nad Ali district of Helmand province, according to Afghan officials.
"The bomb was carried on a motorbike and detonated close to an ISAF patrol," the provincial governor's spokesman Omar Zawak told AFP.
The Taliban claimed responsibility via a text message sent to media organisations.
The last major incident involving NATO troops was 10 days ago, when five US soldiers were killed in an accidental "friendly fire" air strike in Zabul province during clashes with insurgents.
All 50,000 remaining NATO combat troops are due to leave the country by the end of this year, though a small US deployment will remain until the end of 2016 if a long-delayed deal is signed between Washington and Kabul.