"Even if Iraq is in need of help from its brothers and friends in fighting back terrorism, maintaining the sovereignty and independence of its decisions is of the highest importance," Sistani's spokesman Sheikh Abdul Mehdi Karbala'i said during a Friday sermon.
Sistani speaks for millions of Iraqis and has a worldwide following. Earlier, Iraqi authorities said a car bomb blast near a Shiite mosque killed eight people in Baghdad.
Police officials said the attack took place Friday morning when the car exploded near al-Mubarak mosque in the mostly Shiite district of Karradah in downtown Baghdad.
Police said that eight people were killed and 18 others were wounded in the attack. Medical officials confirmed the causalities.
Attacks against Shiite civilians and mosques are common tactics used by Islamic state militants who considers Shiites heretics.
The extremist group has captured large chunks of territory in western and northern Iraq.
Meanwhile, France said its jets had launched strikes inside Iraq for the first time since the country promised to join military action against Islamic State insurgents who have taken over parts of the country.
"This morning at 9:40 (0740 GMT) our Rafale jets launched a first strike against a logistics depot of the terrorists," said a statement from President Francois Hollande's office on Friday shortly after the raids.
Hollande said on Thursday that French air strikes would begin soon, once reconnaissance flights had identified targets.
Islamic State fighters are besieging Kobani, also known as Ayn al-Arab, after seizing at least 21 surrounding villages in a major assault against the predominantly Kurdish city on Syria's northern border with Turkey.
"I call on the international community to use every means as soon as possible to protect Kobani," President Masoud Barzani said in a statement.