A raging wildfire has destroyed around 3,000 hectares of land in a nature reserve in eastern Belgium, close to the German border, as firefighters struggled to get the blaze under control.

A part of the roughly one kilometre long fire front was spreading towards Germany, a spokeswoman for the regional government told dpa on Sunday.

There is however no indication that the fire will cross the border into Germany, she said.

The fire brigade in Germany's Monschau, which lies close to the Belgian border, said most of the approximately 90 emergency personnel had been withdrawn during the night already.

"We currently still have 20 people on standby at the scene," incident commander Philipp Krüger told dpa on Sunday morning.

Authorities in the town of 12,000 said the fire was less than two kilometres from the border but was not expected to spread into German territory.

Around 5,000 residents were told on Saturday to prepare for a possible evacuation. The town spokeswoman said an evacuation was not currently expected, but further updates would follow.

Firefighting helicopters from the Netherlands with large water tanks were being deployed on Sunday morning to help stop the fire from spreading further.

The blaze, which started on Friday, is considered Belgium's biggest wildfire on record, according to local authorities. It had destroyed around 2,700 hectares of land by Saturday evening.

Temperatures dropped overnight in Belgium after a heatwave earlier this week.

Assistance is expected from other European Union countries, following a request by Belgium. Firefighters from Germany and Luxembourg are already deployed at the site as well as the two helicopters from the Netherlands, local authorities said.

The rugged landscape of the High Fens nature reserve is complicating efforts to tackle the blaze, with few roads providing access and extensive peat bogs making firefighting difficult.

The fire changed direction several times during the night due to shifting winds, RTBF broadcaster reported.

On Saturday, around 600 people had to be evacuated from two Belgian municipalities close to the fire.