Washington (CNN) A retired four-star Marine general on Sunday bluntly criticized President Donald Trump over the ongoing Turkish military offensive in northern Syria, saying, "There is blood on Trump's hands for abandoning our Kurdish allies."

"There was no chance (Turkish President Recep Tayyip Erdogan) Erdogan would keep his promise, and full blown ethnic cleansing is underway by Turkish supported militias," he said. "This is what happens when Trump follows his instincts and because of his alignment with autocrats."

"I said there would be blood, but could not have imagined this outcome," added Allen, who previously stressed US military power and a robust armed forces in a 2016 Democratic National Convention speech that endorsed Trump's then rival- Hillary Clinton.

Trump has ordered the remaining US forces out of northern Syria , Defense Secretary Mark Esper said Sunday, a development that has paralyzed the fight against ISIS and ceded US and Kurdish battlefield gains to Moscow and Damascus.

Prior to Turkey's offensive last week, as a confidence building measure with the country, the US convinced the Syrian Kurds to dismantle their defensive fortifications along the border and pull their fighters back. The US said Turkey had agreed to the arrangement which sought to prevent unilateral Turkish military action. Trump then had the Pentagon pull back US troops along that part of the border.

While Kurdish officials and Republican and Democratic lawmakers have argued that the pullback helped provide a de facto green light for the Turkish attack, senior members of the Trump administration have insisted Turkey would have invaded regardless of whether US troops had remained and that the US has not deserted the Syrian Kurds. However, the US government has not taken action yet to stop the Turkish incursion.

The part of Syria that's now under attack has been controlled by the Syrian Kurdish Democratic Union Party (PYD) and the armed Kurdish People's Protection Units (YPG), a defense force mostly made up of Kurds. Now, neighboring Turkey has launched a military operation to move the Kurds away from its border and the frequently targeted ethnic group is once again under attack.

Kurdish forces guarding a US military base in northern Syria told CNN they feel they've been betrayed by their American allies after Trump cleared the way for a Turkish operation against them. The extent of the Turkish military offense has been laid bare in a string of online videos that appear to showcase the violence on the ground -- including footage that reportedly shows Turkish-backed militia fighters shooting Kurdish prisoners.

Allen said Sunday that Trump's approval of $50 million in aid to Syria is a "hollow" gesture.

"Who's going to administer it and for whom? Hundreds of thousands are fleeing and the relief agencies are on the move," he said.

Trump signed an executive order Friday giving the Treasury Department "very significant new sanctions authorities" against Turkey over its actions in Syria, but the US doesn't have any immediate plans to use them, Treasury Secretary Steven Mnuchin told reporters.

A Treasury statement on Friday said Trump's threat of sanctions was meant to dissuade Turkey from actions that included "the indiscriminate targeting of civilians, targeting of civilian infrastructure, targeting of ethnic or religious minorities."