SKOPJE Macedonia has sent about 1,500 migrants and refugees who crossed the border on Monday back to Greece, Macedonian police said on Tuesday.
Most of the migrants, who marched out of the Idomeni transit camp in northern Greece on Monday and got around a border fence, had been taken back to Greece on Monday or overnight on trucks, an official said.
Greece's Deputy Defence Minister Dimitris Vitsas said he could neither confirm nor deny the reports.
Reporters and aid officials on the scene said the migrants had been left at the Greek border and that there were migrants on both the Greek and Macedonian sides of the border.
The 1,500 migrants hiked for hours along muddy paths and forded a rain-swollen river to get around the border fence where they were detained by Macedonian security forces.
About 30 reporters, including a Reuters photographer, were also detained.
A second group of about 600 migrants was prevented from crossing into Macedonia on Monday and many of these spent the night camping in the Greek mountains, according to a Reuters photographer.
At least 12,000 people, including thousands of children, have been stranded in the Idomeni camp, their path to the EU blocked after Macedonia and other nations along the so-called Western Balkan route closed their borders.
European Union leaders are trying to stem a flow of migrants and refugees that brought more than a million people fleeing war and poverty in the Middle East and beyond to the EU since early 2015.
They are due to hold a new summit with Turkey this week to seal an agreement intended to halt the exodus.
Greek Prime Minister Alexis Tsipras said on Tuesday there was "no chance" border shutdowns throughout the Balkans would be lifted and urged refugees to move to reception centres set up by the state.
Jan van't Land, an official with medical charity Medecins Sans Frontieres at Idomeni, said around 400 migrants had returned to Idomeni camp.
"There are still many hundreds of people on both the Greek and the Macedonian side of the border," he told Reuters.
Conditions at the Idomeni camp have deteriorated after days of heavy rain. Scuffles have broken out there in recent days as destitute people scrambled for food and firewood, while many have been sleeping in the open. Concern about the spread of infection grew after one person was diagnosed with Hepatitis A.
Greek officials say leaflets that circulated at the Idomeni camp before Monday's march showed it was planned.
"We are in possession of leaflets that show this was an organised incident, a very dangerous one, endangering people's lives," Greek government spokesman George Kyritsis told reporters on Monday evening.
Babar Baloch, regional spokesman for U.N. refugee agency UNHCR who is at Idomeni, said the migrants' breakout and return "hasn't solved anything."
"It just increased sufferings of refugees. It started raining again. The sense of support for refugees in the region is missing," he said.
(Additional reporting by Ivana Sekularac in Belgrade and Karolina Tagaris in Athens; editing by Adrian Croft/Jeremy Gaunt)