A police chief in Nimroz, Gen. Mohammad Musa Rasouli, was also killed Monday in an explosion in Heart province.
U.S. commander in Afghanistan Gen. John Allen condemned both attacks Monday in a statement.
"This violence against government figures is an affront to peace and makes a mockery of any pretense of the insurgency being part of the future of Afghanistan,” Allen said.
The U.S. embassy in Kabul said that Siddiqi was killed on Human Rights Day.
“This attack, especially on Human Rights Day, shows that those who killed Ms. Siddiqi have no respect for human rights or the safety of the Afghan people,” the embassy said.
The two deaths of government officials come less than a week after the Afghan intelligence chief was wounded in a bombing assassination attempt in Kabul.