Sanliurfa, Turkey — Turkey's president Recep Tayyip Erdogan has threatened to "crush the heads" of America's former allies, the Kurdish-led forces in Syria, if they don't fully withdraw from the Turkey-Syria border by Tuesday evening. The five-day ceasefire that the Trump administration got Erdogan to agree to expires at 7 p.m. local time, or noon Eastern.

Erdogan has said if the Kurds aren't completely out of what he's dubbed a "safe zone," stretching across most of Syria's northern border and about 20 miles south into Syrian territory, his offensive against them will resume. Already it is said to have claimed dozens of civilian lives and has driven hundreds of thousands from their homes.

So as CBS News Holly Williams reports, the stakes were high on Tuesday as Erdogan met President Vladimir Putin in Russia. The future of Syria could be decided at their meeting; Russia is poised to step into the power vacuum the U.S. left behind when President Trump ordered American troops to leave northeast Syria earlier this month, effectively opening the door to a deadly Turkish offensive.

Those U.S. forces only numbered around 1,000, but with their Kurdish partners they were able to beat back ISIS and bring relative stability to a large swathe of Syria after six years of war.

Thousands of the Kurdish-led fighters died battling ISIS, and now say they've been betrayed by America.

About 200,000 civilians have fled the clashes with Turkey, and a Kurdish lawmaker called on President Trump Monday to stop what she called "ethnic cleansing" of the Kurds in northern Syria.

Turkey, however, insists its offensive has not targeted civilians. Erdogan's government views the Kurdish-led forces as terrorists linked to a separatist movement based in southern Turkey.

Syria's Russian-backed President Bashar Assad has lambasted Turkey for its offensive on his soil, and chided the Syrian Kurds for seeking help from the U.S.

On Tuesday the Syrian dictator paid a visit to his troops on the front lines of Idlib province, in territory recently reclaimed from Turkish-backed militias. It was his first visit to the province in seven years.

Arriving in the southern countryside of Idlib, Assad strongly denounced Erdogan for the incursion into northern Syria, calling him "a thief."

"He has robbed the factories, wheat and oil, and today he is robbing the land," Assad asserted.

الرئيس #الأسد: #إدلب كانت بالنسبة لهم مخفرا متقدما، والمخفر المتقدم يكون في الخط الأمامي عادة، لكن في هذه الحالة المعركة في الشرق والمخفر المتقدم في الغرب لتشتيت قوات الجيش العربي السوري.. pic.twitter.com/HVnwD0umRq — Syrian Presidency (@Presidency_Sy) October 22, 2019

Since the U.S. began moving its forces out of the region, the long-time U.S. allies of the Kurdish-led Syrian Democratic Forces (SDF), who for years helped the U.S. battle ISIS while also fending off attacks from Assad's forces, have formed limited partnerships with the Syrian leader's Russian-backed regime.

President Trump has warned Erdogan to restrain his forces and threatened to destroy Turkey's economy if the offensive goes too far. But on Monday, Mr. Trump also said the U.S. had "never agreed to protect the Kurds for the rest of their lives."