Authorities in Greece say thousands migrants and refugees camped out at the country's largest port near Athens have been given two weeks to move to army-built camps or be expelled by force.

The warning issued Thursday came as nearly a third of the 52,000 migrants stranded in Greece by European border closures are refusing to move to organized shelters and remain camped out at the port of Piraeus and at the northern border with Macedonia.

At Piraeus, government officials and translators spent hours trying to persuade migrants to move voluntarily to a new camp in northern Greece, showing them aerial photographs of the site and explaining what facilities are available.

More than 4,000 migrants remain at Piraeus — an important site for Greece's vital tourism industry.