Malaysian activists made a last-ditch legal bid to halt the deportation Tuesday of 1,200 Myanmar detainees to their homeland weeks after a coup, following a storm of criticism.

The migrants, including members of vulnerable minorities, were arriving at a military base on Malaysia's west coast, to be loaded onto three vessels sent by the Myanmar navy.

The United States and the United Nations have criticised the plan, and are calling for the UN refugee agency to be granted access to the detainees to assess whether any are asylum seekers.

The UN says it knows of at least six who are registered with them and in need of international protection.

Rights groups Amnesty International and Asylum Access said they had lodged a challenge Monday at the Kuala Lumpur High Court to stop the deportation.

"This effort to halt the deportation is based on information from refugee groups evidently indicating that asylum seekers and refugees are among the individuals being sent to Myanmar," said Katrina Jorene Maliamauv, executive director of Amnesty International Malaysia.

Activists are making a last-ditch attempt to stop the deportation of 1,200 Myanmar detainees on navy ships
Activists are making a last-ditch attempt to stop the deportation of 1,200 Myanmar detainees on navy ships AFP / Mohd RASFAN

"The Myanmar military's human rights violations against protestors and dissidents have been widely documented. If Malaysia insists on sending back the 1,200 individuals, it would be responsible for putting them at risk of further persecution, violence and even death."

But with the migrants already arriving at the base in Lumut in trucks and buses escorted by police cars, it was unclear what difference the court bid would make.

The Myanmar military seized power at the start of February and detained civilian leader Aung San Suu Kyi, triggering a series of massive protests.

Malaysia initially expressed "serious concern" at the coup -- but just days later, news emerged it had accepted an offer from the Myanmar junta to send warships to repatriate the detainees.

Officials insist those being sent back have committed offences such as overstaying their visas and no members of the persecuted Rohingya minority -- not recognised as citizens in Myanmar -- are among them.

But among the detainees are members of the Christian Chin minority and people from conflict-riven Kachin and Shan states, according to Lilianne Fan, international director of the Geutanyoe Foundation, which works with refugees.

Malaysia is home to millions of migrants from poorer parts of Asia who work in low-pay jobs such as construction. As well as Myanmar, they come from countries including Bangladesh and Indonesia.