The Taliban claimed to have shot down the helicopter, which an Afghan official said crashed in the southern province of Kandahar.
The coalition said it was investigating the circumstances of the crash but gave no other information in a brief statement. It did not release the nationalities of those killed, citing its policy that home countries should identify their dead.
Kandahar provincial police spokesman Zia Durrani said the helicopter went down in the province's Takhta Pul district in the southeast, about 50 kilometers (31 miles) from the Pakistani border. He said five international troops were killed but did not know what caused the crash.
A Taliban spokesman claimed in a text message to journalists that the insurgents shot down the aircraft.
"Today, the mujahedeen hit the foreign forces' helicopter with a rocket, and 12 soldiers on board were killed," Qari Yousef Ahmadi said. The insurgents frequently exaggerate death tolls in their attacks and falsely have claimed responsibility for incidents before.
The last deadliest day for coalition forces was Dec. 17, 2013, when a helicopter crash killed six U.S. service members.
Saturday's deaths bring to seven the number of international troops killed this month. The NATO force is preparing to withdraw its troops from Afghanistan at the end of this year, and it has already turned over the job of fighting the Taliban insurgency to the Afghan army and police.
Violence has increased in Afghanistan ahead of the NATO withdrawal and also in the weeks leading up to the country's April 5 election. Preliminary results of the vote are due later Saturday.
Also Saturday, an Afghan university official identified two Americans killed in a shooting at a Kabul hospital earlier this week.
The vice chancellor of Kabul University, Mohammad Hadi Hadayati, named the two as health clinic administrator Jon Gabel and his visiting father, Gary.
Hadayati said that Jon Gabel's wife was wounded in the attack Thursday saw that an Afghan police security guard open fire as the family entered the grounds of Cure International Hospital.
The Gabel family was visiting pediatrician Dr. Jerry Umanos of Chicago, who was also killed in the shooting.
Hadayati said Jon Gabel ran a clinic at the university providing low-cost medicine and also volunteered to teach computer classes.
He said Gabel worked with U.S. charity Morning Star Development.