Blackwater is the most controversial of several private security firms tasked with protecting high-profile US officials and foreign diplomats in Iraq.
The company's guards shot dead 17 Iraqi civilians while escorting an American diplomat through Baghdad in September 2007 in an incident denounced as a crime by the Iraqi government.
Blackwater says its guards reacted in self-defence.
"As far as Iraqi government is involved, this issue is still under consideration and we are still discussing principles upon which foreign security companies must operate, especially this company because they committed a massacre against Iraqis and until now this matter has not been resolved," Maliki said on Monday in an interview with US television network CNN.
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"No judicial action has been taken, no compensation has been made. Therefore the extension requires approval of the Iraqi government and the government wants to resolve the outstanding issues with this company," Maliki said.
"I would say the US side should not move to renew the contract until the outstanding issues of the company are finalised. I feel the decision was taken without the approval of the Iraqi government."
The US State Department said on Friday it is extending its diplomat protection contract for Blackwater despite the fatal shootings.
The company's contract was set to expire on May 7.
Foreign security companies at currently not subject to Iraq law, but at the same time are not governed by US military tribunals, allowing them to operate without regard to any repercussions.
© 2008 Agence France Presse