Iraq's government on Sunday launched a military operation to track militias blamed for the murder of activists in Basra.
Anti-government protests flared up in the southern oil city last week after gunmen shot dead protest leader Reham Yacoub in her car.
The killing of Yacoub on Wednesday was the third such attack against campaigners in Basra in a week.
Tahseen Oussama, 30, was shot dead on August 14, and four others were shot at while travelling in a car on Monday.
“We will pursue the criminals and arrest the killers within the next few hours,” Interior Minister Othman Al Ghanmi said.
Mr Al Ghanmi said special forces were sent out on Sunday to track those responsible.
The US State Department publicly urged Iraqi Prime Minister Mustafa Al Kadhimi to hold the militias accountable, two days after he sacked the Basra police and national security chiefs and formed a special committee to investigate the attacks.
Events in Basra were in part a result of inaction by the government towards addressing the issues faced by the poor and working class, who are angry and frustrated, said Ali Al Bayati, a member of the Independent Iraqi Human Rights Commission.
"The public feel they are being exploited by some parties for political reasons due to the lack of security in Basra," Mr Al Bayati told The National.
Jeanine Hennis-Plasschaert, special representative of the UN Secretary General and head of the UN mission in Iraq, said the killings were a “serious threat to security and stability” in Basra, the nation’s largest port.
“Basrawis should not live in such an atmosphere of terror and intimidation," Ms Hennis-Plasschaert said last week. "Greater action by the authorities is urgently required.
“The full force of the law must be applied to find, apprehend and hold the perpetrators accountable, and to put an end to this cycle of violence.”
Protesters on Friday burnt the outer gate of the building housing the Iraqi Parliament's local offices for Basra province, the area that produces the largest share of Iraq’s oil.
Demonstrators in Basra have blamed Iran-backed Iraqi militias for the attacks.
Mr Al Kadhimi visited the protest site on Saturday and met security officials shortly after returning from a visit to the US.
He pledged to find the perpetrators and said groups outside the law were trying to terrorise the people of Basra.
The killing of the Basra activists came after Husham Al Hashemi, a security analyst and government adviser, was murdered outside his Baghdad home in July by men on a motorbike.
Mr Al Kadhimi talked tough after Al Hashemi’s killing, pledging to hunt down his assailants and keep armed groups in check.
There have been few developments since then.
When he took office three months ago, Mr Al Kadhimi inherited a country exhausted by decades of sanctions, war, corruption and economic challenges.