By Phil Stewart and Jan Wolfe
WASHINGTON, Oct 11 (Reuters) - The United States ramped up efforts on Friday to persuade Turkey to halt an escalating offensive in northern Syria against U.S.-backed Kurdish forces, saying Ankara was causing "great harm" to ties and could face potentially devastating sanctions.
U.S. President Donald Trump's decision to pull back troops from Syria's border with Turkey has been widely criticized in Washington as a tacit "green light" for a Turkish incursion that experts say could cause a humanitarian catastrophe.
But the Pentagon denied accusations it had abandoned its Syrian Kurdish allies, its strongest partner in the battle against Islamic State, to the Turkish military onslaught.
"Nobody green-lighted this operation by Turkey, just the opposite. We pushed back very hard at all levels for the Turks not to commence this operation," Defense Secretary Mark Esper told a news briefing, accusing Ankara of damaging ties.
Esper and Army General Mark Milley, chairman of the Joint Chiefs of Staff, said they had spoken with their Turkish counterparts repeatedly in recent days, asking them to stop.
But there was no sense that Turkey would do so, they said.
"I'm not seeing any indication or warnings of any planned stoppage of their military activity," said Milley, the top U.S. military officer.
After Milley spoke, President Tayyip Erdogan said Turkey will not stop its operation against the Kurdish militia "no matter what anyone says."
Since it began on Tuesday, the Turkish incursion has opened a new front in the eight-year Syrian civil war and drawn international criticism. A war monitor gave a death toll of more than 100 so far and the United Nations said 100,000 people had fled their homes.
Trump himself has come under withering criticism, including from stalwart Republican backers such as U.S. Senator Lindsey Graham, for withdrawing the U.S. forces whose presence might have prompted Erdogan to hold off on sending in troops.
A senior U.S. State Department official said late on Thursday that the department has been tasked by Trump to see "if there are areas of commonality between the two sides, if there's a way that we could find our way to a ceasefire."
"Our position is that this was a mistake for Turkey to do, that we will try our very best to get this thing stopped," the official said.
Stepping up the pressure, U.S. Treasury Secretary Steven Mnuchin appeared at the White House on Friday to announce that Trump had authorized U.S. officials to draft "very significant" new sanctions to target Turkey, but the conditions under which they would be applied were unclear.
Mnuchin said the United States was not activating the sanctions at this time but would do so if necessary.
"We are putting financial institutions on notice that they should be careful and that there could be sanctions," he said. "These are very powerful sanctions. We hope we don't have to use them, but we can shut down the Turkish economy if we need to."
Mnuchin said Trump was concerned about any potential targeting of civilians by Turkish forces, and wants to make clear that Turkey cannot "allow even a single ISIS fighter to escape."
In its first big attack since the assault began, Islamic State claimed responsibility for a deadly car bomb in Qamishli, the biggest city in the Kurdish-held area of northeast Syria, even as the city came under heavy Turkish shelling.
Five Islamic State fighters fled a jail there, and foreign women from the group being held in a camp torched tents and attacked guards with sticks and stones, the Kurds said.
Milley told reporters he had just spoken with the chief of the Turkish General Staff, General Yasar Guler, who he said "understands clearly" that the prisoners would be Ankara's responsibility in the areas where it carries out the offensive.
"We, the U.S. military, have no responsibility to ... secure those ISIS prisoners in Syria," Milley said.
Pentagon officials also stressed the need for Turkey to avoid doing anything to endanger U.S. forces inside Syria, who numbered about 1,000 prior to the incursion. Although U.S. troops had no intention of firing on Turkey, its North Atlantic Treaty Organization ally, the Pentagon noted they had the right to defend themselves.
Milley said Turkey had been told of U.S. positions in Syria.
"The Turkish military is fully aware - down to explicit grid coordinate detail - of the locations of U.S. forces," he said.
An explosion occurred on Friday near a U.S. military outpost near Kobane in Syria, a U.S. official said, but no U.S. personnel were reported hurt and the source of the blast was not immediately clear.
Turkey's Defence Ministry said it took all measures to ensure no U.S. base was damaged while it responded to harassment fire that originated near a U.S. base in Syria. (Additional reporting by Makini Brice, Jeff Mason, Idrees Ali and Humeyra Pamuk in Washington, and Ali Kucukgocmen in Istanbul; Editing by Mary Milliken and Daniel Wallis)