WASHINGTON (Reuters) - The United States will mount diplomatic pressure on Turkey and threaten Ankara with more sanctions to persuade it to halt its military offensive in northeastern Syria, a senior administration official said on Tuesday.

A week after reversing U.S. policy and moving troops out of the way to allow Turkey to attack Washington’s Syrian Kurdish allies, President Donald Trump announced a package of sanctions to punish Ankara, but Turkey ignored the penalties and pressed on with its assault on Tuesday.

Speaking at a briefing, the U.S. official, who declined to be identified, said Turkey’s incursion caused “a mess” in what he said once was a relatively stable part of northern Syria. A delegation from Washington, led by Vice President Mike Pence, was going to Ankara within 24 hours for further talks, he said.

“The plan is to continue the pressure on Turkey as we evaluate our chances to return the relationship to normal, a major element of that return to normal would be a ceasefire,” the senior administration official said.

“And by ceasefire what I mean is forces on the ground stop moving on the ground. Certainly Turkish forces and I think we could probably speak for the SDF,” he said, referring to Washington’s former Kurdish allies.

The United States announced on Sunday it was withdrawing its entire force of 1,000 troops from northern Syria. The official said the United States had now pulled out all forces from the Syrian town of Manbij, once a flashpoint where Turkish and U.S. troops carried out joint patrols. The official added Washington still controlled the airspace in the region.

As the United States pulled out, the SDF immediately forged a new alliance with President Bashar al-Assad’s Russia-backed government, inviting the army into towns across the breadth of its territory.

The official did not provide further details about Pence’s trip but said if Ankara did not respond to diplomatic efforts, there could be more sanctions.

“We’re planning to increase sanctions and other measures many of which have been signaled to the administration, absent a resolution to this crisis,” the official said.

The measures announced on Monday - mainly a hike in steel tariffs and a pause in trade talks - were less robust than financial markets had anticipated, and Trump’s critics derided them as too feeble to have an impact.

‘THUGS, BANDITS, PIRATES’

Prisons holding captive Islamic State fighters in northeastern Syria have had security difficulties, the senior administration official said, but no major prison breaks have occurred.

Ankara’s unilateral offensive has also angered Turkey’s main European NATO allies who fear a return of Islamic State in the region. European countries are especially concerned about foreign Islamic State fighters and adults returning to Europe.

Europeans comprise a fifth of around 10,000 Islamic State fighters held captive in Syria by Kurdish militias, and their wives and children are often in Syria too, living in camps.

The United States has seen human rights violations carried out by Turkey-backed rebels, the U.S. official said, and even though the actions were not carried out by the Turkish troops, Washington views Ankara as responsible.

“They took a relatively peaceful situation...and turned it into this absolute mess you are looking at... They could have used Turkish, regular troops, instead they decided to use these thugs, bandits and pirates that should be wiped off the face of the earth,” he said.

On Saturday, the Syrian Observatory for Human Rights, a UK-based organization that reports on the war, said Turkey-backed groups had killed nine civilians, including Hervin Khalaf, secretary general of the Future Syria Party. Washington said it was disturbed by the reports.

“One group identified with the killings along the M4 (highway) is a well-known jihadi element, just short of being designated as a terrorist organization,” the official said. It was not immediately clear if he specifically referred to the killing of the Kurdish politician.