Flying in from Jordan, Kerry met with Prime Minister Nuri al-Maliki and other Iraqi leaders to urge a speeding up of the government formation process in order to face down the insurgents.
The United States' "support will be intense, sustained, and if Iraq's leaders take the steps needed to bring the country together, it will be effective," Kerry told journalists in Baghdad.
But the danger to Iraq, he said, is dire. "It is a moment of decision for Iraq's leaders," Kerry said. "Iraq faces an existential threat and Iraq's leaders have to meet that threat."
During their talks, Maliki also emphasised the danger of the crisis, telling Kerry it "represents a threat not only to Iraq but to regional and international peace," his office said.
Iraqi security forces are struggling to hold their ground in the face of an insurgent onslaught that has seized major areas of five provinces, displaced hundreds of thousands of people and sparked fears the country could be torn apart.
Maliki's security spokesman said today "hundreds" of soldiers had been killed since the insurgents, led by the powerful jihadist Islamic State of Iraq and the Levant (ISIL), launched their offensive on June 9.
The announcement on television by Lieutenant General Qassem Atta is the most specific information provided so far by the government on losses sustained by the security forces.
The militants are continuing their charge, overrunning the Al-Waleed border crossing with Syria, officers said today. The capture of the post means all official crossings with Iraq's neighbour to the west are outside government control, and increases the militants' ability to bring men and materiel across the border from Syria.
Insurgents also overran the strategic Shiite-majority northern town of Tal Afar and its airport, an official and witnesses said today.