More than 1,300 foreign troops have been killed in Afghanistan since 2001
Four US soldiers have been killed by a bomb in southern Afghanistan, a Nato spokesman has said.
Brig Gen Eric Tremblay said they were struck "while patrolling in one of the most violent areas of Afghanistan", but did not give an exact location.
The deaths bring the number of foreign forces killed in Afghanistan in 2009 to 295, the deadliest year since the US-led invasion to overthrow the Taliban.
The previous deadliest year was 2008, when 294 military personnel died.
The soldiers whose deaths were announced on Tuesday had been operating under the command of the Nato-led International Security Assistance Force (Isaf).
"It is with great sorrow that I learned about this terrible attack and I wish to convey my sincere condolences to the families and loved ones of these four determined Isaf comrades," said Gen Tremblay.
"We shall always be proud of their courage and sacrifice as they fell fighting for the good of Afghans living in the south," he added.
US and Nato casualties in Afghanistan are fast mounting. At least 63 foreign military personnel have been killed this month, including 37 Americans. A total of 1,340 have died in the country since 2001.
More than 30,000 extra US troops have been sent to Afghanistan since President Barack Obama ordered reinforcements in May, almost doubling his country's contingent and increasing the Western total to about 100,000.
On Sunday, the chairman of the US Joint Chiefs of Staff, Admiral Mike Mullen, said the security situation in Afghanistan was "serious and deteriorating" and left open the possibility of further reinforcements.
"Afghanistan is very vulnerable in terms of [the] Taliban and extremists taking over again, and I don't think that threat's going to go away," he told CNN.
"The Taliban insurgency has gotten better, more sophisticated."
The latest casualties come five days after Afghanistan held presidential and provincial elections. Partial preliminary results from the presidential poll are due shortly.
Representatives for the two leading candidates, President Hamid Karzai and Abdullah Abdullah, have already claimed to be in the lead.