Pakistan on Monday resumed its mass repatriation of Afghan refugees despite past accusations of coercion in the supposedly voluntary UN programme to return hundreds of thousands to a war-torn nation.

The operation, which saw 380,000 registered refugees sent back from Pakistan in 2016, was halted in December for a routine winter break.

"The UNHCR voluntary repatriation programme for registered Afghan refugees resumed today," Duniya Aslam Khan, a spokeswoman for the refugee aid body said.

The UN had cut its cash grant for returnees from USD 400 to USD 200, she confirmed, citing "financial constraints facing humanitarian operations worldwide".

The grant was doubled in 2016 and became a factor in the surge of returnees across the border to Afghanistan after July last year, the UN has said.

But fears of a crackdown by Pakistan on refugees, many of whom left Afghanistan decades ago, also contributed.

Human Rights Watch in a scathing report in February accused Pakistan of coercion, threats and abuse in the mass repatriation, and the UN of complicity.

The report said a combination of insecure legal status, the threat of deportation during winter and police abuses — including extortion, arbitrary detention and night raids — had left the Afghan refugees with no choice but to leave.

It also accused UNHCR of effectively encouraging the exodus by doubling the cash grant, and said the UN body should end the "fiction" that the returns are voluntary.

The UN has previously rejected the criticism.

"UNHCR advocates for all refugee returns from Pakistan to be voluntary, gradual and take place in conditions of safety and dignity," Khan said Monday.

The refugees are returning to an Afghanistan still torn apart by decades of war. They will compete for resources with a record half a million people who were internally displaced by the fighting in 2016, according to UN figures.

The mass migrations are draining local resources, especially in safer urban areas, officials have said.

Some 4.2 million Afghan refugees have returned to Afghanistan voluntarily under the UNHCR-funded Voluntary Repatriation programme since 2002.

But UNHCR and Pakistani officials have said some 1.34 million registered refugees still live in Pakistan.

A further half a million undocumented refugees are also estimated to still be in the country, making Pakistan one of the world's largest refugee-hosting nations