A bomb exploded near Kabul on Tuesday, killing 16 people, including four NATO troops and two Afghan police officers.

The blast went off at a clinic in Bagram, near the largest U.S. base in Afghanistan. Ten civilians were also killed by the bomb, the BBC reported.

A suicide bomber was behind the attack, according to Associated Press.

The blast came amid fears of a political crisis in Afghanistan. Preliminary results of the country’s presidential election showed former Finance Minister Ashraf Ghani winning the contest, but rival Abdullah Abdullah claimed the process was rife with fraud and proclaimed himself the winner.

The situation led U.S. Secretary of State John Kerry to warn Afghanistan it would lose security and support if anyone tried to take control of the country “by extra-legal means,” according to the BBC. He also cautioned against a power grab following reports that Abdullah was planning on instituting a parallel government.