Russian military units have been sent to patrol near the Syria-Turkey border, sparking fears Moscow is moving to fill a security vacuum after U.S. troops started withdrawing from the region last week.

On Tuesday, a U.S. military spokesman said American troops had officially left the town of Manbij as part of their withdrawal from northeast Syria.

Col. Myles B. Caggins tweeted: “We are out of Manbij.”

He added that the U.S.-led “coalition forces were executing a deliberate withdrawal” from the area.

A U.S. official told the Associated Press that the approximately 1,000 U.S. troops will be repositioned in Iraq, Kuwait and possibly Jordan. The Manbij region is home to U.S. outposts that were set up in 2017 to patrol the tense frontiers between Turkish-controlled areas and the Kurdish-held side of northern Syria.

The news came as Russian military units were sent to the region to police “along the line of contact between the Syrian Arab Republic and Turkey,” a Russian Defense Ministry statement said, according to The Washington Post.

Turkey has thus far defied the growing condemnation from its NATO allies to press ahead with its invasion of northern Syria, shelling suspected Kurdish positions near the border amid reports that Syrian Kurds had retaken Ras al-Ayn, a key town.

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Turkish media reports said Turkey's military was responding to attempts by the Kurdish fighters to infiltrate Ras al-Ayn.

Meanwhile, a Russian envoy for Syria said Moscow will not allow Turkish and Syrian government forces to clash, underscoring his country’s role as the de facto power broker in the conflict.

Alexander Lavrentyev, a presidential envoy for Syria, told Russian state news agencies on Tuesday that “no one is interested” in potential fighting between Syrian government troops and Turkish forces that entered Syria last week. He said the Kremlin “is not going to allow it.”

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On Monday, Syrian fighters backed by Turkey had said they had started an offensive to capture Manbij, which is on the western flank of the Euphrates River, broadening their campaign east of the river. But a Turkish military official denied those reports, without giving further detail.

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The United States and other western allies of Turkey have condemned the operation, warning that it could lead to a resurgence of the Islamic State group in the region. The Trump administration, on Monday, called on Turkey to stop the invasion and declare a cease-fire amid announcing sanctions aimed at retraining the Turks’ assault against Kurdish fighters.

Trump is sending Vice President Mike Pence and national security adviser Robert O'Brien to Ankara as soon as possible in an attempt to begin negotiations. Pence said Trump spoke directly to Turkish leader Recep Tayyip Erdogan, who promised not to attack the border town of Kobani, which in 2015 witnessed the Islamic State group's first defeat in a battle by U.S.-backed Kurdish fighters.

"President Trump communicated to him very clearly that the United States of America wants Turkey to stop the invasion, implement an immediate cease-fire and to begin to negotiate with Kurdish forces in Syria to bring an end to the violence," Pence said.

Trump said the approximately 1,000 U.S. troops who had been partnering with local Kurdish fighters to battle the Islamic State in northern Syria are leaving the country. They will remain in the Middle East, he said, to "monitor the situation" and to prevent a revival of ISIS.

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Defense Secretary Mark Esper said Monday he would travel to NATO headquarters in Brussels next week to urge European allies to impose "diplomatic and economic measures" against Turkey — a fellow NATO ally — for what Esper called Ankara's "egregious" actions.

Esper said Turkey's incursion had created an unacceptable risk to U.S. forces in northern Syria and "we also are at risk of being engulfed in a broader conflict."

Meanwhile, Erdogan has not given any indication that he will halt the offensive, saying on Tuesday, that Turkish troops will “soon secure the region from Manbij to the border with Iraq.”

He defended Turkey's offensive in an op-ed in The Wall Street Journal, calling on the international community to support Turkey's effort to create what it calls a resettlement "safe zone" for refugees in northeast Syria, or "begin admitting refugees."

"Turkey reached its limit," Erdogan wrote in reference to 3.6 million Syrian refugees in his country. He said Turkey's warnings that it would not be able to stop refugee floods into the West without international support "fell on deaf ears."

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The United Nations has said that as many as 160,000 people, including 70,000 children, have been displaced since the fighting in northeast Syria escalated nearly a week ago.

The Associated Press contributed to this report.