Iraq's Prime Minister Nouri Maliki has rejected calls for a national salvation government to help counter the offensive by jihadist-led Sunni rebels, according to BBC News.

Such calls represented a "coup against the constitution and an attempt to end the democratic experience", he warned.

The U.S. 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 U.S. military advisers assigned to help the Iraqi security forces have arrived and are to start work on Wednesday, June 25.

Meanwhile, the crisis in Iraq is being discussed by NATO leaders meeting in Brussels. They have been joined by U.S. Secretary of State John Kerry, who has just returned from a two-day visit to Baghdad and Irbil.

In his weekly televised address, 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, which has long accused him of discrimination and monopolizing power. He rejected forming an emergency administration including all religious and ethnic groups, arguing that such a move would go against the results of April's parliamentary elections which were won by his bloc.

"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."

During his visit to Iraq, Kerry warned that Iraq 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 U.S. military advisers are setting up a joint military operations room with the Iraqi army in Baghdad and another in the north.

U.S. 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. The U.S. intelligence assessment is that the Sunni rebels spearheaded by ISIS are capable of holding the territory they have captured. Iraqi forces have tacitly recognized that. They have been unable to launch any strategic counter-offensives, the BBC says.

They are mainly focusing on two things - harassing the rebels from the air, mainly with attack helicopters, and building up their deployment for the defense of Baghdad, where troop numbers have been doubled.