Nauru’s government has ordered Médecins Sans Frontières (MSF) to stop work there immediately, despite a physical and mental health crisis in the country.

The medical NGO has been providing psychological and psychiatric services to residents, asylum seekers and refugees on the island since late 2017.

MSF staff wrote to its clients on Saturday to tell them it could no longer offer them treatment because of an order from Nauru’s health minister, who said the its services were “no longer required on the island”.

MSF said in its letter that the order meant its “mental health activities in Nauru must stop, effective today, Saturday, 6th of October”.

“We know this sudden news may be very difficult to hear and we deeply regret any challenges that this presents to you, all of our patients, and to the community.”

The organisation said it was seeking more information from the government, including the reason for its decision and what alternative services would be available.

An MSF spokeswoman said later on Saturday that the government had asked on Friday that its activities stop within 24 hours.

“At this stage MSF wishes to reiterate our strong commitment to providing quality mental health care to all those in need on the island,” she said. “We are extremely concerned that the health of our patients may be affected by this decision and urge the authorities to grant us permission to continue our lifesaving work.”

The government could not be immediately be reached for comment.

There have been growing calls for children and their families to be removed from the island to receive treatment. Some have already been quietly moved off the island, including to Australia, but about 90 children remain, according to reports from last month.

Dozens have been transferred to Australia under federal court order, including for high-level mental health treatment as the crisis worsens among all age groups. The Australian authorities have argued against transfer recommendations in a number of cases – spending $320,000 on doing so in the last financial year – but has complied with orders.

Late last month, however, Guardian Australia reported that the country’s authorities had been unable to send an air ambulance to Nauru for a court-ordered medical transfer because the island’s government refused permission.

According to MSF’s website, it has provided “one door for all” psychological and psychiatric services as part of an agreement with Nauru’s health ministry.

It has pointed to a lack of mental health support for both “the Nauruan population and the asylum seekers and refugees living on the island as part of the Australian government’s policy of offshore processing”.

MSF staff were also training and supporting local authorities in an effort to increase Nauru’s capacity to provide psychological and psychiatric treatment.

Ian Rintoul, a spokesman for the Refugee Action Coalition, said Nauru’s decision was because MSF was “perceived to be helping refugees”, but that the Australian government was to blame: “The offshore detention monster created by Australia is indeed out-of-control.

“The deliberate distress of five years of offshore detention has created the medical emergency that we are currently witnessing. The deliberate harm has created a humanitarian disaster.”

A spokeswoman for Australia’s interior ministry said the issue was a matter for Nauru’s government.

“The Department of Home Affairs takes seriously its role in supporting the government of Nauru to ensure that refugees and asylum seekers are provided with a range of health, welfare and support services arrangements. This has not changed.

“The Australian government has, and continues to provide appropriate healthcare and mental health support to refugees and asylum seekers through contracted service providers.”