Hungary was due to begin detaining asylum-seekers in shipping-container camps on its southern border with Serbia on Tuesday.

Asylum-seekers entering Hungary as well as those already in the country will be confined in camps while their applications are processed.

A year after tighter border controls sealed off the Balkan route for people seeking asylum in Europe, many continue to stream into Serbia where a bottleneck has formed due to tough security measures to the north. Mostly of Afghan and Pakistani origin, 6,500 asylum seekers are currently in Serbia, a thousand of whom have occupied an abandoned train depot in Belgrade.

A statement by the interior ministry said the country's prison service installed 324 shipping container homes at two camps.

"The border protection agencies are fully prepared for the entry into force of the reinforced legal border closure on March 28," said a statement by the interior ministry.

"The purpose of the restrictions is to prevent migrants with an unclear status from moving freely around the territory of the country and the European Union, and to thereby reduce the security risk of migration," the statement said.

Only unaccompanied children under the age of 14 were exempted from mandatory detention in the border camps.

Detainees would receive access to beds, bedding, personal hygiene packages, lockers, continuous hot water, toilets, mass media and telecommunication equipment and religious rooms, according to the statement.

They would receive three meals a day (five meals a day for children under the age of 14) and fruit and dairy products for expectant mothers, mothers with young children and children under the age of 14 years, according to the statement.

The United Nations and rights groups such as Amnesty International condemned Hungary for failing to meet Hungary's international obligations to asylum-seekers when it passed a law earlier this month that led to the camps' development.

Prime Minister Viktor Orban, who took a hard-line anti-immigration stance, said the bill was in response to recent terror attacks in Europe carried out by migrants.

Hungary once detained all asylum applicants but ended the practice in 2013 under pressure from Brussels, the UN refugee agency and the European Court of Human Rights.

The UNHCR said systematic detention would "have a terrible physical and psychological impact on women, children and men who have already greatly suffered".

Refugee rights group the Hungarian Helsinki Committee said 400 asylum-seekers housed in the country's internal camp network faced relocation to the border camps.

A second "smart fence" complete with night cameras, heat and movement sensors, and multilingual megaphones warning against crossing the barrier was also under construction, with completion scheduled by May.

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