John Kerry said he was confident the government of Iraqi PM Nouri Maliki could defeat the militants.
Earlier, the Iraqi government said it had lost control of the strategic city of Fallujah, west of Baghdad.
Al-Qaeda-linked militants now control the south of the city.
An Iraqi reporter there says tribesmen allied with al-Qaeda hold the rest of Fallujah.
John Kerry made the comments as he left Jerusalem for Jordan and Saudi Arabia to discuss his effort to broker peace between Israel and the Palestinians.
“We will stand with the government of Iraq and with others who will push back against their efforts to destabilize,” he said.
“We are going to do everything that is possible. I will not go into the details.”
He added: “We are not contemplating putting boots on the ground. This is their fight.”
The head of the police in Anbar province – Hadi Razeij – said on Saturday that his men had pulled back to the city walls and the people of Fallujah were “the prisoners of Isis”.
Fighting there erupted after troops broke up a protest camp by Sunni Arabs in the city of Ramadi on Monday.
They have been accusing the Shia-led government of marginalizing the Sunnis.
Local Sunni Arabs have been angered by what they perceive as discrimination by the government of Nouri Maliki.
They also say their minority community is being targeted by anti-terrorism measures implemented to stem the surge in sectarian violence.
In recent months, Sunni militants have stepped up attacks across Iraq, while Shia groups began deadly reprisals – raising fears of a return to full-scale sectarian conflict.
wpsg_XBOQkoImage source: Wikimedia Commons President-elect Donald Trump celebrated his election victory at the Ultimate Fighting…
Millions of voters across the US chose to return Donald Trump to the White House…
Donald Trump declares victory in the US election as he addresses jubilant supporters in Florida.…
Stocks around the world are rising as Donald Trump appears to be on the cusp…
Donald Trump has won Pennsylvania, North Carolina and Georgia and taken a lead over Kamala…
Quincy Jones, the celebrated musician and producer who worked with Michael Jackson, Frank Sinatra, Ray…