JACKSONVILLE - President Barack Obama on Monday mourned 14 Americans killed in helicopter crashes in Afghanistan, saying they died to keep Al-Qaeda out of the war-torn country.

"With today’s helicopter crashes in Afghanistan, fourteen Americans gave their lives," Obama told servicemen and women at a naval air station in Jacksonville, Florida.

"Our prayers are with these service members, their civilian colleagues and the families who loved them," said Obama who flew to the event after holding his latest White House war council on future strategy in Afghanistan.

"While no words can ease the ache in their hearts today, may they find some comfort in knowing this: like all those who give their lives in service to America, they were doing their duty and they were doing this nation proud.

"They were willing to risk their lives, in this case, to prevent Afghanistan from once again becoming a safe haven for Al-Qaeda and its extremist allies. And today, they gave their lives ... to protect ours."

NATO earlier said 14 Americans were killed and more than 25 other people injured in two military helicopter crashes in Afghanistan on Monday, one of them an apparent mid-air collision, NATO said.

The deaths made Monday one of the deadliest single days in Afghanistan for foreign civilian and military personnel working to rebuild the country and defeat a Taliban insurgency, eight years after the US-led invasion.

One helicopter crashed in western Afghanistan, NATO's International Security Assistance Force (ISAF) said in a statement, adding: "The cause is not believed to be from enemy action."

Ten Americans were killed and 26 other people wounded in the incident.

Another four US service members were killed and two other soldiers wounded in an apparent mid-air collision between two helicopters in southern Afghanistan, NATO said in an earlier statement.