The journal quoted US officials as saying Vienna, where talks on limiting Iran's nuclear capabilities take place, could be used as a diplomatic channel to sound out mainly Shiite Iran which has long backed Iraqi Prime Minister Nouri al-Maliki.

Over the weekend, Iran's president, Hassan Rouhani, said Tehran might "think about" cooperating with Washington to confront Sunni ISIS fighters, whose advance appears to have halted north of Baghdad.

The US State Department said on Sunday its number two diplomat, Bill Burns, would travel to Vienna this week.

Iran's help needed?

Almost simultaneously, US Republican Senator Lindsey Graham told the US television program "Face the Nation" that Maliki could not keep Iraq together and Washington would need Iran's involvement to fend off the ISIS advance.

"We are probably going to need their help to hold Baghdad," Graham said, adding that ignoring the problem in Iraq could lead to another 9/11 attack.

State Department spokeswoman Jen Psaki said the US would boost security at its Baghdad embassy - America's largest worldwide - and would evacuate some of its staff.

Australia followed suit, saying only an "essential core" of its embassy staff would remain.

At the weekend, President Barack Obama ordered an aircraft carrier into the Gulf.

Alarm over film sequence

Unverified photos posted online on Sunday, ostensibly by the ISIS, purported to show its fighters summarilyexecuting dozens of captured Iraqi security force members. Most of those captured were wearing civilian clothes.

Other photos purported showed ISIS fighters seizing facilities on Wednesday in Tikrit, the hometown of the late Sunni strongman Saddam Hussein who was removed in a 2003 US-led invasion.

Residents flee Tal Afar

Residents in the northwestern Iraqi city of Tal Afar said Sunni insurgents had seized it amid heavy casualties and leaving "families trapped inside houses."

A city official quoted by the news agency Reuters said the city of mainly ethnic Turkmen had been "overrun by militants."

"Shiite families have fled to the west and Sunni families have fled to the east," said the official, referring to Iraq's complex ethnic and sectarian mix.

Tal Afar is a short drive west of Mosul, northern Iraq's main city which ISIS fighters seized early last week and lies near Iraq's autonomous northern Kurdish region.

Kurdish security forces used the collapse of central government control during the past week to into Kirkuk and adjacent rural areas with oil deposits.

Analysts say Iraq's army, supposedly a million strong, which was trained and armed by Washington at a cost of $25 billion (18.5 billion euros) has been plagued by corrupt and factional interests.

A call by Iraq's top Shiite cleric resulted at the weekend in thousands of volunteers flocking toward Baghdad to join security forces.

US-led forces that invaded Iraq in 2003 withdrew in 2011.

ipj/kms (Reuters, AFP, AP)