Turkey’s assault on Syrian Kurdish forces has not breached a red line declared by President Donald Trump, a US official said Thursday, as he added that Washington wanted to broker a ceasefire.

Turkish ground forces pressed their advance against Kurdish fighters in northern Syria on Thursday, Turkey’s Defense Ministry said, launching airstrikes and unleashing artillery shelling on Syrian towns and villages the length of its border.

The U.S. military has no plans to intervene if Syrian Kurdish forces abandon a constellation of Islamic State prisons in Syria to confront a possible Turkish invasion, officials said Tuesday.

Turkey said on Tuesday it had completed preparations for a military operation in northeast Syria after the United States began pulling back troops, opening the way for a Turkish attack on Kurdish-led forces long allied to Washington.

In a major shift in United States military policy in Syria, the White House said on Sunday that President Trump had given his endorsement for a Turkish military operation that would sweep away American-backed Kurdish forces near the border in Syria.