Top US military brass warned Monday it would be a long haul to peace in Afghanistan following an explosion there that killed three civilians.

“This is going to be a long, windy, bumpy road. There will be ups and downs, and we’ll and stop and start,” Defense Secretary Mark Esper told reporters at the Pentagon.

The blast in Khost— which also injured 11 — came just two days after the US and the Taliban signed a peace agreement that President Trump said would lead to the withdrawal of US troops after nearly 19 years of fighting.

The agreement followed a seven-day “reduction in violence” meant to test the Taliban’s seriousness about moving toward a final agreement.

Esper said he had approved the start of an American troop withdrawal even as the Taliban — which denied responsibility for Monday’s explosion — said it would continue attacks against Afghan forces.

Esper said he wasn’t sure if the drawdown had begun, but he said it must start within 10 days of the US-Taliban peace agreement. He said Gen. Scott Miller, the US commander in Kabul, would begin the withdrawal of about 8,600 troops from the current total of nearly 13,000, and then would pause and assess conditions.

“Our expectation is that the reduction in violence will continue,” said Esper, adding that officials will work to determine who is responsible for any acts of violence.

With Post wires