The unusual decision comes after four Americans, two soldiers and two contractors, were killed in a suicide attack Saturday on the Bagram Airfield near the capital, Kabul.

Two days earlier, insurgents attacked the German Consulate in the northern city of Mazar-i-Sharif, killing six people and wounding more than 100.

The US Embassy in Kabul regularly warns Americans against travel to Afghanistan, where a Taliban-led insurgency is in its 16th year.

The insurgency has become more virulent since most international combat troops withdrew in 2014.