Airstrikes may be an option in Iraq, says Kerry
Mr Kerry also said that he would not rule out potential US-Iranian military cooperation in stemming the advance of Sunni extremists.
Critically, he also baldly stated that US drone strikes "may well" be an option.
Mr Kerry said Washington is "open to discussions" with Tehran if the Iranians can help end the violence and restore confidence in the Iraqi government.
Asked about possible military cooperation with Iran, Mr Kerry said he would "not rule out anything that would be constructive". However, he stressed that any contacts with Iran would move "step-by-step".
US officials said earlier that a senior American diplomat would also discuss Iraq with an Iranian delegation at nuclear talks in Vienna.
Mr Kerry spoke as it emerged that cold-blooded executions said to have been carried out by jihadists in northern Iraq almost certainly amount to war crimes, the United Nations said yesterday, as a key northern town fell to the insurgents.
After the Islamic State of Iraq and al-Sham (Isis) released graphic photographs of its fighters shooting scores of young men, Navi Pillay, the UN High Commissioner for Refugees, voiced shock over the bloodshed.
Isis claimed to have executed 1,700 people after capturing the Iraqi city of Tikrit.
Ms Pillay said the figure could not be verified, but added: "This apparently systematic series of cold-blooded executions, mostly conducted in various locations in the Tikrit area, almost certainly amounts to war crimes."
She urged "comprehensive action", saying: "We want to alert the world to address this immediately."
Isis advanced further yesterday, seizing the town of Tal Afar in north-western Iraq. Inhabited largely by the Turkmen ethnic minority, Tal Afar was held up as a symbol of the success of US forces when they expelled insurgents from the area in 2005.
Its loss marks another defeat for Iraq's armed forces.
It lies halfway between Mosul, the biggest prize seized by the rebels last week, and the Syrian border, continuing Isis's efforts to establish an Islamic caliphate across both countries.
Isis claimed to have captured Tal Afar's commanding general, Khaled Abu Walid al-Khazraji, and said it would execute him publicly in the town square of Mosul. But that was denied by the Iraqi ministry of defence, which quoted him saying he was "in good health" and safe.
Isis also later took the town of Saqlawiya west of Baghdad, with bodies of the dead left lying in the street. Fighters took away more American-supplied military equipment.
According to officials, the rebels now control two airports, three airstrips and 30 military bases across the country, including ones once well-known as centres of the American occupation. Isis has shown little sign of worrying about the international response to its actions. Its attempts to terrify its enemies continued yesterday with more postings on social media of executions of men it captured on its march south.
The Tikrit killings were "horrifying and a true depiction of the bloodlust that these terrorists represent", the US State Department spokesman, Jen Psaki, said.
The US could not confirm the reports, she added, but one of the primary goals of Isis was "to set fear into the hearts of all Iraqis and drive sectarian division among its people".
Corroborating video posted online yesterday seemed to show similar sequences of young men being led away to their fate. Ms Pillay said that among other atrocities the militants had also executed 13 imams in Mosul for refusing to pledge allegiance to Isis.
One bloody set of new photographs posted on the Facebook page of a fighter will cause particular outrage.
It showed a Tunisian jihadi with five Iraqi Shia soldiers kneeling at his feet. He demands that they denounce Iraq's Shia prime minister, Nouri al-Maliki. One of the five who refuses is shot dead on the spot. The next photograph shows the other four have also been killed, left in pools of blood. (© Daily Telegraph, London)