Sunni militants say they have seized a strategically important border crossing and two towns along the Euphrates river in continued fighting in Iraq.
Officials admitted that the militants had seized the border crossing to Syria near the town of Qaim, killing 30 troops after a day-long battle.
Rebels also said they had taken the towns of Rawa and Aneh.
Correspondents say a campaign along the river may eventually lead to an assault on Baghdad from the west.
The capture of the Qaim crossing in western Iraq could help Isis transport weapons and other equipment to different battlefields, analysts say.
Sunni extremists have seized control of large areas of territory across Iraq in recent days.
They claim to have seized parts of Iraq’s largest oil refinery, at Baiji, and have also taken a disused chemical weapons factory in Muthanna, 70km (45 miles) north-west of Baghdad.
On Saturday the government again denied that militants had gained access to parts of the Baiji refinery but did admit the army was facing “violent attacks” from gunmen.
Also on Saturday, thousands of Shia militia loyal to the powerful cleric Moqtada al-Sadr paraded through the streets of Baghdad.
The cleric, whose Mehdi Army fought the US in Iraq for years, had called for a military parade across the country.
Correspondents say the show of force will be seen as a very disturbing development by the Baghdad government.
The impressive-looking parade of men in battle fatigues accompanied by serious military hardware will only raise sectarian tensions at at time when the government is under pressure to rally the country together against the extremists, our correspondent reports.
US Secretary of State John Kerry is expected to travel to Iraq soon to press for a more representative cabinet, hoping this could ease tensions between the country’s rival Muslim sects.
Meanwhile, US President Barack Obama said Isis – which has an estimated 10,000 fighters in Iraq and Syria – had exploited a power vacuum in Syria to amass arms and resources, but denied this was because the US had not moved to back moderate rebel forces fighting President Bashar Assad.
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 in the face of Iraqi calls for US air strikes, the White House is insisting that there is no purely military solution to the crisis.
Correspondents say Mr Obama believes Iraqi PM Nouri Maliki has endangered the country by ignoring Sunni concerns and governing in the interests of the Shia majority.
The UN estimates that about one million people have been displaced within Iraq as a result of violence this year.
About 500,000 people fled their homes in the country’s second-largest city, Mosul, which Isis captured last week.