Macedonian police official said on Tuesday that the country has turned about 1,500 refugees back to Greece.
The refugees had passed the border on Monday and Macedonia turned them back to Greece overnight by trucks, said the official, who asked for anonymity.
There were no official statement from the Macedonian government yet.
On the Greek part, Deputy Defence Minister Dimitris Vitsas said on Tuesday that he could not confirm or deny whether refugees, who left a Greek transit camp and crossed into Macedonia, had been sent back to Greece.
"What I know is that any procedure will take place via the usual diplomatic means," said Vitsas.
"I'm not aware of anything further in order to confirm or deny this."
About 1,500 of refugees marched out of a Greek transit camp on Monday to get around a border fence and cross into Macedonia, where they were detained by Macedonian security forces.
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 UN refugee agency UNHCR who is at Idomeni, said the refugees' 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.
About 30 reporters, were also detained.
Meanwhile, another group of refugees on early Tuesday left from a Greek transit camp and walked for hours in muddy paths in order to cross into Macedonia.
According to Macedonian police, three refugees drowned while trying to cross a river close to the Greek border.
At leas 12,000 people remain in the village of Idomeni since Macedonia built a fence on the border to change the refugees' route.
Europetan Union leaders are trying to stem a flow of refugees that brought more than a million people fleeing war and poverty in the Middle East and beyond to the EU since early 2015.
The leaders are planning to hold a new summit with Turkey this week to reach an agreement intended to stop the influx.
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.
An official from MSF (Doctors Without Borders) said around 400 refugees had returned to Idomeni camp.
"There are still many hundreds of people on both the Greek and the Macedonian side of the border," he said.
Conditions at the Idomeni camp are worsening day by day after days of heavy rain. Reportedly, refugees are complaining about lack of clean water, food, dress and medical care, while concern about the spread of infection has grown after one person was diagnosed with Hepatitis A.