Iraq’s Prime Minister Nouri Maliki has rejected calls for a national salvation government to help counter the offensive by jihadist-led Sunni rebels.
Such calls represented a “coup against the constitution and an attempt to end the democratic experience”, he warned.
The US has led appeals to the country’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 80km (50 miles) north of Baghdad.
Also on Wednesday, a suicide bombing outside the main market in the northern city of Kirkuk left at least two people dead and many more injured.
The city was seized by Kurdish peshmerga fighters on 12 June when the Iraqi army fled in the face of the rebel advance.
At least nine people were also killed in attacks in the town of Mahmudiyah to the south of Baghdad.
Meanwhile, the crisis in Iraq has been discussed by Nato leaders meeting in Brussels. They have been joined by US Secretary of State John Kerry, who has just returned from a two-day visit to Baghdad and Irbil.
He has announced he will be going to Saudi Arabia on Friday to hold further talks on the crisis.
Mr Kerry said Mr Maliki was “following through” on commitments to move forward on the process of government formation.
In his weekly televised address, Mr 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).
Mr 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.”
Mr Maliki committed to start forming a new governing coalition by 1 July.
Speaking to the BBC in Dubai, UK Defence Secretary Philip Hammond criticised the current Iraqi government’s “deficit of inclusiveness”.
During his visit to Iraq, Mr Kerry warned that the country was now at a “critical moment” in its history. He promised that US support “will be intense and sustained and if Iraq’s leaders take the necessary steps to bring the country together, it will be effective”.
The 130 US military advisers are setting up a joint military operations room with the Iraqi army in Baghdad and another in the north.
US officials have made it clear that this is not a “rush to the rescue”, although the US advisers are in the position to call in air strikes against the militants if it is deemed necessary.
Their primary job is to assess the capabilities of the Iraqi forces and advise on what should be done, says the BBC’s Jim Muir in Irbil.
The US intelligence assessment is that the Sunni rebels spearheaded by Isis are capable of holding the territory they have captured.