Taliban suicide attackers killed the South African leader of a foreign aid group, his son and daughter and an Afghan worker in an assault on the agency's Kabul offices, authorities said Sunday, as the city's police chief resigned in the wake of the latest insurgent attack there.

Police chief Gen. Mohammed Zahir told reporters before his resignation that the attack Saturday actually killed four people, raising the death toll in the assault near the Afghan parliament. He offered no other details about the victims and did not name the aid group.

A Redlands, California-based group called Partnership in Academics and Development later posted a notice on its website saying several of its staffers died during an attack Saturday in Kabul.

"We are caring for all staff and their families as they grieve the loss of their friends and co-workers and nurse the wounded," the statement read. "Our thoughts are with the survivors and their families as they grieve the loss of life. Their selfless sacrifice for the people of Afghanistan is an inspiration to all."

A message left at a phone number listed for the group was not immediately returned Sunday. Clayson Monyela, a spokesman for South Africa's foreign affairs department, said Sunday that he had no information about the slain South Africans.

Saturday's attack saw three Taliban militants launch an assault on the office, with one exploding a suicide bomb vest and the two others later killed in a shootout with police, authorities said. At least one of the attackers wore a police uniform, Zahir said.

Police spokesman Hashmat Stanikzai later said Zahir had resigned his post, without providing further details.

Kabul has seen eight deadly suicide attacks against high-profile targets in the past 16 days, one of the most violent periods in the capital in years. In recent days, four foreigners -- including an employee of the British embassy -- have been killed, and dozens of Afghan civilians have been killed and wounded.

The attacks have raised concerns about whether Afghan security forces can protect the country after the U.S. and NATO officially conclude their 13-year combat mission on Dec. 31. They also show a reinvigorated Taliban insurgency taking advantage of the situation as well.

President Ashraf Ghani late Sunday signed into law security agreements with Washington and NATO that will keep around 12,000 troops in the country in 2015 after parliament approved them.

He condemned the recent attacks in Kabul and elsewhere as "unacceptable" and said the security pacts marked "an important step forward in the development of our country."

Analysts say the agreements likely are the reason for the escalated Taliban attacks.