An Iraqi Shiite masked militiaman, a follower of Shiite cleric Muqtada al-Sadr, holds up his machine gun during a parade in the northern oil rich province of Kirkuk, Iraq, Saturday, June 21, 2014. Thousands of heavily-armed Shiite militiamen paraded through several Iraqi cities on Saturday as Sunni militants seized two strategically located towns in what appeared to be a new offensive in the western Anbar province. Photo: AP
Sunni militants have seized another town in Iraq's western Anbar province - the fourth in two days.
Fighters of the Islamic State in Iraq and the Levant (Isis) captured Rutba, 90 miles (150km) east of Jordan's border, officials said.
They earlier seized a border crossing to Syria and two towns in western Iraq as they advance towards Baghdad.
The insurgents intend to capture the whole of the predominantly Sunni Anbar province, a spokesman told the BBC.
Meanwhile, Iran's supreme leader Ayatollah Ali Khamenei has said he "strongly opposes" US intervention in Iraq.
"The main dispute in Iraq is between those who want Iraq to join the US camp and those who seek an independent Iraq," he said, adding that Washington was simply seeking to keep Iraq within its own sphere of power.
The capture of the frontier crossing could help Isis transport weapons and other equipment to different battlefields, analysts say.
The rebels are confident that towns they do not already control along the Euphrates valley will fall without much of a fight, with the help of sympathetic local tribes, says the BBC's Jim Muir in Irbil.
Since January, they have held parts of the provincial capital Ramadi, and all of nearby Falluja, half an hour's drive from Baghdad.
A spokesman for the Military Councils, one of the main Sunni groups fighting alongside Isis, told the BBC the rebels' strategic goal was the capital itself.
In the meantime they are clearly trying to take the string of towns along the Euphrates between Falluja and the western border, says our correspondent.
There is deep pessimism in Baghdad about the government's war against Isis, which appears better trained, equipped and more experienced than the army, diplomats and politicians have told the BBC.
The Sunni extremists attacked the city of Mosul in early June and have since seized swathes of territory across Iraq.
The Iraqi government has urged the US, Europe and the UN to take immediate action to help deal with the crisis - including targeted air strikes.
Iraq's air force ran out of American Hellfire missiles two weeks ago, and only has two Cessna planes capable of firing the missiles.
But Isis has established secure safe havens, including some in neighbouring Syria, which will be difficult to target, experts say.
And experts warn that using air strikes now would endanger civilians.
"[The militants are] now fully enmeshed with the civilian population and it's just almost impossible to use air power or cruise missiles to strike at fighters that way," said Christopher Harmer, an analyst from the Institute for Study of War in Washington.
"You will end up killing a lot of civilians," he told the BBC.
The US, which pulled out of Iraq in 2011, is sending some 300 military advisers to Iraq to help in the fight against the insurgents there.
But the White House insists there is no purely military solution to the crisis.
Obama believes Iraqi PM Nouri Maliki has endangered the country by ignoring Sunni concerns and governing in the interests of the Shia majority, correspondents say.