Bulgaria'a army stepped in on Tuesday to buttress the southwestern border with Macedonia, which has been hit in recent days by an unprecedented influx of migrants.

Twenty-five soldiers and light armoured vehicles will be deployed at Bulgaria's four border checkpoints with Macedonia, a defence ministry operations and training deputy chief told public BNR radio.

"Our task will be to reinforce the controls already implemented by border police," Nikolay Karaivanov said.

He said the number of soldiers might be increased at a later stage if necessary.

Asked to provide details about the tasks to be performed at the border, the defence ministry's public affairs chief Daniel Stefanov said the soldiers "will perform training-related tasks far from the immediate border line, inside our territory."

"We do not envisage joint patrols with border police," he said, adding that the equipment will include mostly armoured sports utility vehicles and trucks rather than armoured personnel carriers, and no tanks will be sent.

Until now Bulgaria has focused on reinforcing control of its southeastern border with Turkey by sending over 1,000 extra police as well as expanding a 30-kilometre (20-mile) barrier to prevent migrants from entering.

But an influx of thousands of refugees entering neighbouring Macedonia from Greece over the past weeks has prompted the country to shift its focus.

Still, Interior Minister Rumyana Bachvarova estimated on Tuesday that the risk of seeing a migrant influx similar to that in Macedonia was "relatively low."

"Everyone knows that the Bulgarian borders are guarded very well," Bachvarova told public BNR radio.

Migrants who crossed the Macedonia-Greece border are now streaming into European Union member state Hungary, with a record 2,093 crossing over from Serbia on Monday.

They are heading north towards the wealthier economies of the EU.