Authorities say the blast Saturday in the Afghan capital, Kabul, killed at least 12 people and wounded 66.
NATO did not name the dead contractors in its statement.
The attack struck near the private Shinozada hospital in the capital’s Macrorayan neighborhood just as schools were letting out.
The Taliban denied carrying out the attack. However, they have stepped up their assaults on Afghan security forces since U.S. and NATO troops ended their combat mission in the country last year.