Islamist militants fighting in Iraq released a video on Tuesday which purported to show the beheading of American journalist James Foley and US officials said they were working to determine its authenticity.
The video, titled "A Message to America," was released a day after Islamic State, an al Qaeda offshoot that has overrun large parts of Iraq, threatened to attack Americans "in any place."
"We have seen a video that purports to be the murder of US citizen James Foley by ISIL," White House National Security Council spokeswoman Caitlin Hayden said in a statement. "The intelligence community is working as quickly as possible to determine its authenticity."
"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends," she said.
Islamic State militants also claimed in the video to be holding US journalist Steven Sotloff and said his life depended on US President Barack Obama's next move. The video was posted after Obama resumed air strikes in Iraq for the first time since the end of the US occupation in 2011.
"The life of this American citizen, Obama, depends on your next decision," said a masked man in the video posted on social media sites, speaking English with a British accent as he held a prisoner the video named as Sotloff, who went missing in northern Syria while he was reporting in July 2013.
Foley was kidnapped on Nov. 22, 2012, in Syria by unidentified gunmen.
A Twitter account set up by Foley's family said on Tuesday: "We know that many of you are looking for confirmation or answers. Please be patient until we all have more information, and keep the Foleys in your thoughts and prayers."
In another video on Tuesday, Islamic State spoke of a holy war with the United States and said it would emerge victorious over the "crusader" America. It showed footage of Obama and King Abdullah of Saudi Arabia, as well as attacks on US soldiers.
Earlier, Iraqi forces halted a short-lived offensive on Tuesday to recapture Tikrit, home town of executed dictator Saddam Hussein, due to fierce resistance from Islamic State fighters.
Buoyed by an operation to recapture a strategic dam from the militants after two months of setbacks, Iraqi army units backed by Shi'ite militias launched their offensive shortly after dawn on Tikrit, a city 130 km (80 miles) north of Baghdad which is a stronghold of the Sunni Muslim minority.
But officers in the Iraqi forces' operations room said by mid-afternoon that the advance had stopped.
South of Tikrit, the government side came under heavy machinegun and mortar fire from the militants, a group of Arab and foreign fighters hardened by battle both in Iraq and over the border in Syria's civil war, the officers told Reuters.
To the west, landmines and snipers frustrated efforts to get closer to the city centre in the latest in a series of attempts to drive out the militants. Residents of central Tikrit said by telephone that Islamic State fighters were firmly in control of their positions and patrolling the main streets.
Sunni Muslim fighters led by the Islamic State swept through much of northern and western Iraq in June, capturing the Sunni cities of Tikrit and Mosul as well as the Mosul Dam, a fragile structure which controls water and power supplies to millions of people down the Tigris river valley.
On Monday, fighters from Iraq's Kurdish autonomous region said they had regained control of the hydro electric dam with the help of US air strikes. Obama announced that the dam had been retaken.
The Pentagon said US fighter aircraft conducted two air strikes near the Mosul Dam in the last day. One destroyed an Islamic State checkpoint.
Obama ordered the US air strikes earlier this month, he said, to protect Americans and prevent a genocide in a conflict that has forced hundreds of thousands of Iraqis to flee their homes, including from the Yazidi and Christian religious minorities.
The Islamic State has concentrated on taking territory for its self-proclaimed caliphate both in Syria, where it is also fighting the forces of President Bashar al-Assad, and in Iraq. Unlike al Qaeda, the movement from which it split, it has so far steered clear of attacking Western targets in or outside the region.
Coinciding with the Kurdish advances, Damascus government forces have stepped up air strikes on Islamic State positions in and around the city of Raqqa – its stronghold in eastern Syria.
Analysts believe Assad - who is firmly in control in the capital more than three years into the civil war - is seizing the moment to show his potential value to Western states that backed the uprising against him but are now increasingly concerned by the Islamic State threat.
The Islamic State added new fighters in Syria at a record rate in July, according to the Syrian Observatory for Human Rights, which monitors the conflict. About 6,300 men – 80 percent of them Syrian and the rest foreigners – joined last month, Rami Abdelrahman, founder of the Observatory, told Reuters.
Germany Weighs Arms
The Islamic State's successes since June have alarmed governments both in the West and in the region.
German Foreign Minister Frank-Walter Steinmeier said on Tuesday that Germany would decide this week whether to send arms to Iraqi Kurds, suggesting it might be irresponsible to do nothing.
Saudi Arabia's Grand Mufti Sheikh Abdulaziz Al al-Sheikh, the highest religious authority in the country, said on Tuesday that the Islamic State and al Qaeda were "enemy number one of Islam" and not in any way part of the faith.
Efforts are under way in Baghdad to form a new government that will unite the majority Shi'ites with the Sunnis and Kurds in halting the Islamic State insurgency that threatened to tear the country apart.
In Geneva, the United Nations refugee agency announced a major aid operation to get supplies to more than half a million people displaced by fighting in northern Iraq.