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Greece has finished work on a 40-kilometre border fence with Turkey that Athens hopes will prevent a surge of illegal migration from Afghanistan.
A new surveillance system is also in place to catch potential asylum seekers fleeing the Taliban.
Hundreds of thousands of people have been forced out of their homes during fighting in Afghanistan, triggering fears of a new refugee crisis.
Those concerns are especially acute in Greece, which was on the frontline of the migrant wave in 2015 when more than a million people entered Europe.
Greek Migration Minister Notis Mitarachi said Greece would not accept being the “gateway for irregular flows into the EU” after the fall of Kabul.
He said Greece regarded Turkey, which already hosts millions of refugees, as a safe place for Afghans to live.
Turkish President Recep Tayyip Erdogan said Ankara “has no duty, responsibility or obligation to be Europe’s refugee warehouse”.
The new border wall, a cement and barbed-wire extension to an existing 13-kilometre fence, was completed in recent days. Construction began after a dispute with Turkey last year.
Civil Protection Minister Michalis Chrisochoidis visited the Evros region where the wall was put up on Friday.
“We cannot wait, passively, for the possible impact,” he said. “Our borders will remain safe and inviolable.”
Already under pressure over a separate migration crisis in Belarus, Europe has increasingly turned to military might to defend its borders.
EU leaders want to help Afghanistan’s neighbours intercept migrants before they reach Europe. Austria called at a summit this week for millions of euros in funding to go to nearby countries.
In a call with Greece’s prime minister on Friday, Mr Erdogan said Ankara was in contact with Iran to discuss the issue of Afghan migrants.
Iran and Pakistan are home to many Afghan refugees, while Uzbekistan and Tajikistan have both increased security at their borders.
