*Regime returns to northeast Syria
*US senators express concern
*Trump, EU warn of sanctions

Turkey will not back down from its offensive against Kurdish fighters in northeastern Syria "no matter what anyone says", President Tayyip Erdogan said Monday, adding that the battle would continue until "ultimate victory" is achieved.
"We are determined to continue the operation until the end, without paying attention to threats. We will absolutely finish the job we started. Our battle will continue until ultimate victory is achieved," Erdogan said during a speech in Baku.
He also slammed the European Union and Arab League for their criticism of Turkey's operation and asked for international funds for Ankara's "safe zone" plans in northeast Syria.
Meanwhile, US Senate Republican leader Mitch McConnell expressed grave concern about the US response to Turkey's incursion in Syria, and said he will meet with officials in President Donald Trump's administration and other senators this week to discuss the situation.
"Abandoning this fight now and withdrawing US forces from Syria would re-create the very conditions that we have worked hard to destroy and invite the resurgence of ISIS," he said in a statement in Washington, referring to the Islamic State militant group. "And such a withdrawal would also create a broader power vacuum in Syria that will be exploited by Iran and Russia, a catastrophic outcome for the United States' strategic interests."
AFP adds from Tall Tamr, Syria: The Syrian regime deployed troops near the Turkish border and entered a key city on Monday to contain Ankara's offensive against the Kurds, stepping in for US forces due to begin a controversial withdrawal.
The army has kept a presence in Kurdish-controlled Qamishli and Hasakeh in Syria's northeast since the 2011 outbreak of the country's war, and deployed a limited number of troops around the key city of Manbij last year at the request of Kurdish forces.
Their new deployment, notably inside Manbij, marks the regime's return to a region from which Damascus started to withdraw in 2012 and a significant gain for President Bashar al-Assad, who has vowed to reclaim all of Syrian territory.
Outgunned and without US protection, the autonomous Kurds had few other options to stop the rapid advance of Turkish troops and their Syrian proxies.
Turkey wants to create a roughly 30-km buffer zone along its border to keep Kurdish forces at bay and also to send back some of the 3.6mn Syrian refugees it hosts.
The United States and its partners, who spent years fighting alongside the Kurds against the Islamic State group in Syria before deserting them, have condemned the Turkish invasion, but their threats of sanctions have failed to stop it.
"Big sanctions on Turkey coming!" US President Donald Trump said Monday.
Washington says it is planning to pull out 1,000 troops -- almost the entire ground force -- from Syria's north, in a move welcomed by Turkish President Recep Tayyip Erdogan as "a positive approach"
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