QUETTA, Pakistan - Pakistan on Monday opened three new border crossings to accelerate the repatriation of undocumented Afghan nationals who have been ordered to leave the country or face expulsion, officials said.

Many Afghans have opted to go home voluntarily to avoid deportation under a government push for undocumented migrants to be expelled. Pakistan's move affects more than 1 million Afghans, many of whom Islamabad says have been involved in militant attacks and crime, a claim Kabul rejects.

The new crossings were set up at the Afghan border in southwestern Balochistan province in addition to the main crossing in Chaman district, said Jan Achakzai, information minister for the provincial caretaker government.

The main crossing had been overwhelmed with Afghan refugees seeking to return home voluntarily, he said.

More than 280,000 Afghan nationals have left Pakistan since the new policy was announced in early October, according to the United Nations High Commissioner For Refugees (UNHCR).

Islamabad has begun round-up operations across the country after the deadline for voluntary departure expired on Nov. 1.

Pakistan has so far rejected calls from the United Nations, rights groups and Western embassies to reconsider its expulsion plan or to identify and protect Afghans who risk persecution at home.

Kabul has also asked Islamabad to give Afghan nationals ample time to leave.

The expulsion drive has driven relations between the neighbours to a new low, with Islamabad reiterating its claim that Islamist militants use Afghan soil to plan and carry out attacks in Pakistan. Kabul says Pakistan's security is a domestic problem.

The mass migration has also raised fears of a humanitarian crisis as Kabul grapples with hundreds of thousands of people arriving and staying in makeshift tent villages on its side of the border at the onset of winter.

(Writing by Asif Shahzad; Editing by Hugh Lawson)