Hungarian prisoners have finished building the first part of a second anti-migrant fence on Hungary's southern border with Serbia.

Prime Minister Viktor Orban announced the plans for a second fence line in August to reinforce the existing barrier erected last year along the 175-kilometre-long border.

The risk that the European Union's deal earlier this year with Turkey to stem the flow of migrants might not last means Hungary needs to strengthen its border defences, Orban said.

Orban has emerged as the standard-bearer of those opposed to German Chancellor Angela Merkel's "open-door" policy, in the wake of Europe's worst migration crisis since World War II.

Dubbed a "smart fence" by local media due to its high-tech features, construction by prisoners of the three-metre-high (10-feet) and 10.3-kilometre-long barrier began a month ago.

The new fence is equipped with heat and movement sensors, as well as night cameras installed at regular intervals, according to media reports.

When fully built it is expected to cut the numbers of troops and police officers required to patrol the border, said the M1 pubic television news channel.

The government will decide later on when the rest of the fence line will be built, said the channel.

In 2015 over 400,000 migrants trekked through Hungary, a border of the EU's passport-free Schengen zone, toward northern Europe.