An explosion in Afghanistan, claimed by the Taliban on Monday, killed three U.S. Marines and injured another three.

The three men died after they hit a roadside bomb near their Bagram Air Base, north of Kabul. “We feel and mourn the loss of these Americans with their families and loved ones,” said Gen. Scott Miller, Commanding General of Resolute Support and U.S. Forces Afghanistan. “They volunteered to protect their country. We will continue our mission.”

Military authorities initially believed the attack had killed a fourth man, but they later determined he was alive and receiving medical treatment alongside the other casualties.

“The contractor who was reported as killed, is alive,” the U.S.-led military operation in Afghanistan, Operation Resolute Support, said in a corrective statement on Tuesday. “The contractor, an Afghan citizen, was initially treated along with other injured civilians, later identified as a contractor and treated at Bagram Airfield.”

The attack may further undermine the ongoing U.S.-brokered peace talks with the Taliban that U.S. officials hope will conclude before Afghanistan’s presidential elections in November. The talks would potentially bring an end to an 18-year-war that has been a focal point of U.S. foreign policy.

The Trump administration is reportedly planning to withdraw at least half of the 14,000 troops stationed in Afghanistan. Under recent plans drawn up by the White House, all American military will be withdrawn from the country over the next three to five years in exchange for a deal that allows the current government in Kabul to share power with the Islamist organization.

Last week, Trump claimed that troop withdrawals were an indication of “progress” in the negotiations. “As we make progress in these negotiations, we will be able to reduce our troop presence and focus on counter-terrorism,” he said. “We do not know whether we will achieve an agreement — but we do know that after two decades of war, the hour has come to at least try for peace.”

Having started in October 2001, the war in Afghanistan has come at an enormous cost for the United States, both in lives and financial investment. The U.S. is estimated to have spent $1 trillion, equivalent to at least $3 billion per month, leading to 2,278 U.S. military deaths, and a further 20,426 injuries.

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