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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.
Iraq's government said on Sunday it had killed 40 militants in an air strike on the militant-held northern town of Tikrit, although witnesses said civilians died when a petrol station was hit.
US Secretary of State John Kerry, speaking in Cairo, said Isis' "ideology of violence and repression is a threat not only to Iraq but to the entire region".
Calling it a "critical moment", he urged Iraq's leaders "to rise above sectarian motivations and form a government that is united in its determination to meet the needs and speak to the demands of all of their people".
Earlier, Iran's supreme leader Ayatollah Ali Khamenei condemned the prospect of US intervention in Iraq, saying Washington's main intention was to keep Iraq within its own sphere of power.
Dismissing talk of sectarianism, he said: "The main dispute in Iraq is between those who want Iraq to join the US camp and those who seek an independent Iraq."