KABUL, Afghanistan (AP) — Afghan officials on Monday mourned the loss of 21 soldiers killed by the Taliban in the single deadliest incident for the Afghan army in at least a year, as new details emerged about the attack which threatened to further strain relations with neighboring Pakistan.

The soldiers died Sunday when hundreds of heavily armed Taliban insurgents attacked two remote army checkpoints near to each other in eastern Afghanistan’s Kunar Province. Officials said that five Afghan soldiers are missing.