Such calls represented a “coup against the constitution and an attempt to end the democratic experience”, Nouri al-Maliki warned.
The US has led appeals to Iraq’s political leaders to rise above sectarian and ethnic divisions.
Government forces have been unable to recapture the territory seized by the rebels this month.
Almost half of the 300 US military advisers assigned to help the Iraqi security forces have arrived.
Fighting was reported to have continued on Wednesday, with an attack by rebels on the Balad airbase, about 50 miles north of Baghdad.
In his weekly televised address, Nouri al-Maliki called on “all political forces to reconcile” in the face of a “fierce terrorist onslaught”.
But the Shia prime minister gave no promise of greater representation in government for the minority Sunni Arab community, whose anger at what they say are his sectarian and authoritarian policies has been exploited by jihadist militants from the Islamic State in Iraq and the Levant (ISIS).
Nouri al-Maliki said forming an emergency administration that included all religious and ethnic groups would go against the results of April’s parliamentary elections, which were won by his State of Law alliance.
“The dangerous goals of forming a national salvation government are not hidden,” he said.
“It is an attempt by those who are against the constitution to eliminate the young democratic process and steal the votes of the voters.”
Nouri al-Maliki committed to start forming a new governing coalition by July 1st.
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