DEIR AL-ZOR, Syria/BEIRUT (Reuters) - U.S.-backed Syrian militias on Tuesday relaunched their offensive to seize the last territory Islamic State controls in the east near the border with Iraq.

Slideshow ( 4 images )

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, had paused the battle after Turkey launched an assault in January against their northwestern Afrin region.

“We have rearranged our ranks,” said Lilwa al-Abdallah, spokeswoman for the offensive in oil-rich Deir al-Zor province.

Islamic State militants stepped up attacks there in recent weeks in a bid to reorganize, she told a news conference at an oilfield on the eastern bank of the Euphrates river. “Our heroic forces will liberate these areas and secure the border...We welcome the support of the Iraqi forces.”

Ahmed Abu Khawla, commander of the Deir al-Zor military council fighting under the SDF, said they were working with the Baghdad government and Iraqi army “through a joint operations room” to defeat the jihadists.

Joint efforts had increased, but neither side would cross the border, he told Reuters.

U.S. Defense Secretary Jim Mattis said last week that he expected a “re-energized” effort soon against the ultra-hardline militants in eastern Syria.

Syrian fighters, backed by U.S. air strikes and troops, have dealt heavy blows to Islamic State but the jihadists still hold a swathe of land along the desert frontier with Iraq. They are widely expected to revert to guerrilla tactics if they lose the last remnants of their once self-styled “caliphate”.

The SDF alliance, spearheaded by the Kurdish YPG militia, has seized vast tracts of territory from Islamic State in north and east Syria. U.S. support for Kurdish forces there has infuriated Turkey, which sees the YPG as an extension of an outlawed Kurdish insurgency at home.

Ankara’s offensive to expel the YPG from Afrin - where the United States has no presence - led to a pause in the campaign against Islamic State, the Pentagon has said.

Arab SDF militias redeployed 1,700 fighters from eastern fronts against Islamic State to help fight Turkish forces, which then captured Afrin in March.

Abu Khawla said those SDF forces have now returned to the east. The assault on Afrin “distracted from eliminating Daesh”, he said, using the Arabic acronym for Islamic State.

“It had a very big and wide impact on the liberation, which was stalled for months.”