A homicide attacker on a motorbike exploded himself next to highway police guarding a road construction project in the Afghan capital Wednesday, killing one policeman and a civilian, officials said.

An SUV carrying foreigners that had its window shattered in the attack may have been the intended target, said Gen. Ali Shah Paktiawal, the Kabul police director of criminal investigations. The SUV drove off and officials didn't know what group or country it was from.

One policeman and one civilian were killed in the blast, said Asid Arian, a district government official. One woman and one child were injured, he said.

The attack came on one of Kabul's busiest and most dangerous roads, where more homicide and rocket attacks happen than in any other part of the city.

Attacks in Afghanistan have increased the last several weeks. More than 1,800 people have been killed in insurgency-related violence this year, according to an Associated Press count based on U.S., NATO and Afghan officials.