Refugee boys carry their belongings during a police operation at a refugee camp at the border between Greece and Macedonia, near the village of Idomeni, Greece, 24 May 2016. [Photo/Agencies]

IDOMENI -- Greek police started moving migrants and refugees out of a sprawling tent camp on the sealed northern border with Macedonia on Tuesday where thousands have been stranded for months trying to get into western Europe.

Reuters witnesses saw several bus loads of migrants leaving the makeshift camp of Idomeni early on Tuesday morning, with about another dozen buses lined up. It appeared to be mainly families who were on the move.

Greek authorities said they planned to move individuals gradually to state-supervised facilities further south in an operation expected to last several days.

"The evacuation is progressing without any problem," said Giorgos Kyritsis, a government spokesman for the migrant crisis.

A Reuters witness on the Macedonian side of the border said there was a heavy police presence in the area but no problems were reported as people with young children packed up huge bags with their belongings.

Media on the Greek side of the border were kept at a distance and a group of people dressed as clowns waved balloon hearts and animals as the buses drove past.

"Those who pack their belongings will leave, because we want this issue over with. Ideally by the end of the week. We haven't put a strict deadline on it, but more or less that is what we estimate," Kyritsis told Reuters.

At the latest tally, 8,199 people were camped at Idomeni after a cascade of border shutdowns throughout the Balkans in February barred migrants and refugees from central and northern Europe. More than 12,000 lived in the camp at one point.

The International Rescue Committee's country director, Panos Navrozidis, said that on-site pre-registration had proved a good incentive for refugees to leave Idomeni although the asylum process remained "inadequate and slow".

Railway tracks between Greece and Macedonia have been blocked by migrants for weeks, forcing trains to switch routes through Bulgaria further to the east. Some goods wagons have been stranded on the tracks for weeks.