NATO is working to secure the site of a helicopter crash in central Afghanistan, where 38 people, including nearly two dozen members of the elite U.S. Navy SEALS, were killed when their aircraft was reportedly shot down by insurgents.

Saturday's crash in Wardak province was the deadliest incident for U.S. forces since the start of the decade-long war.

The Chinook helicopter went down during an anti-Taliban operation in the Tangi Valley. Thirty U.S. troops, including seven Afghan soldiers and an Afghan interpreter were killed.

In a statement released late Monday, NATO said the helicopter was reportedly fired on by an insurgent rocket-propelled grenade while transporting U.S. service members and commandos to the scene of an ongoing battle between coalition forces and insurgents.

NATO says the operation began as a security search for a Taliban leader, and that forces on the ground requested additional troops to assist in the operation. The reinforcements were in-bound to the scene of the fighting when the helicopter crashed.

The coalition says an investigation is under way to determine the exact cause of the crash. NATO spokesman Brigadier General Carsten Jacobsen told reporters Monday that the site was being secured and that nobody was being allowed into the area while the probe is ongoing.

U.S. officials says 22 of the dead were Navy SEAL commandos, including members of SEAL Team 6, the unit that killed al-Qaida leader Osama bin Laden in Pakistan in May. Officials say those killed in the helicopter crash did not take part in the bin Laden mission.

NATO spokesman Jacobsen says that despite the “tragic loss,” the military campaign against insurgents will continue and the enemy will be “relentlessly pursued.”

Earlier on Monday, another coalition helicopter made a hard landing in the eastern Afghan province of Paktia. NATO said there were no casualties and that there were no initial reports of enemy activity in the area. The cause of the incident was being investigated.

Late Sunday, Afghan President Hamid Karzai called his U.S. counterpart, Barack Obama, to reiterate his condolences for the tragic loss of 30 American troops in Wardak. A White House statement said Mr. Obama noted the extraordinary service of the Americans who gave their lives, and expressed his condolences for the Afghans who died serving with them.

The two sides also reaffirmed their commitment to the mission in Afghanistan, which they said was critical to the security of both countries.

Violence in Afghanistan is at its worst since the U.S.-led invasion in 2001, with international troop and Afghan civilian deaths reaching record levels.