The motive for the attack in the west of Kabul was not clear and police were investigating who was responsible, Bashary said.
AFP later reported a Taliban spokesman claiming his group undertook the attack. Zabihullah Mujahid said: "We killed her because she was working for an organisation which was preaching Christianity."
There have been three assassinations in the southern city of Kandahar in the past week also carried out by men on motorbikes.
The Taliban claimed responsibility for two of the killings involving the city's most senior policewoman and a government official.
The third was of a tribal elder shot dead with his son, a former bodyguard of Hamid Karzai, the Afghan president. They had left a mosque early in the morning.
An NGO security group said last week attacks on aid workers by fighters in Afghanistan were at the highest level in six years.
There has been 146 security incidents involving non-government organisations till September this year compared to 135 for the whole of 2007, the Afghanistan NGO Safety Office (Anso) said in a quarterly report.
They included 28 killings, including those of five international workers, and 72 abductions, the group said.