Greece has completed a 40km (25-mile) fence and surveillance system on its border with Turkey amid concerns over a surge of migrants from Afghanistan.
"We cannot wait, passively, for the possible impact," Greece's Citizens' Protection Minister Michalis Chrisochoidis said on a visit to the region of Evros on Friday.
"Our borders will remain inviolable."
His comments came as Turkey called on European countries to take responsibility for Afghan migrants.
In a telephone conversation with Greek Prime Minister Kyriakos Mitsotakis, Turkey's President Recep Tayyip Erdogan said a sharp increase in people leaving Afghanistan could pose "a serious challenge for everyone".
"A new wave of migration is inevitable if the necessary measures are not taken in Afghanistan and in Iran," Mr Erdogan said.
The rapid takeover of Afghanistan by the Taliban, an Islamist militant group, has left some fearing for their lives and seeking to escape the country, often by any means necessary.
Mr Chrisochoidis said the crisis had created new "possibilities for migrant flows" into Europe.
Greece began extending the fence at the border with Turkey last year, after initially installing a 12.5km fence in the region in 2012.
The country, which was on the frontline of the migrant crisis in 2015 when more than a million people fleeing war and poverty in the Middle East crossed from Turkey into the EU, has said it may send back any Afghans that arrive illegally through the country.
Of those who arrived in Greece during the migrant crisis, many travelled further north throughout Europe, but about 60,000 have remained in the country.
Last year, Athens temporarily blocked new asylum applications after Mr Erdogan said Turkey had "opened the doors" for migrants to travel to the EU.
Mr Mitsotakis said at the time that Greece had increased "the level of deterrence at our borders to the maximum", with security personnel deployed to the Evros land border.
Correction, 31 August 2021: This article has been amended to make clear that work to extend an existing fence has been completed.