The United Nations refugee agency said Tuesday that it had raised concerns with Australian authorities that a boatload of Vietnamese asylum seekers had their refugee applications rejected at sea before the Australian navy secretly returned them to Vietnam.

The 46 Vietnamese were offloaded from an Australian navy ship at the port city of Vung Tau last Friday, local Australian media reports said.

UNHCR spokeswoman Vivian Tan said Australian officials had confirmed that asylum seekers had been screened at sea and returned to Vietnam.

The UNHCR had asked the Australian government on Monday for details of the procedures used to assess their applications, Tan said. Australia had yet to respond.

"We've expressed concerns before about the procedures of screening at sea because we're worried that it may mean that people seeking asylum may not have gone through the proper procedures, their protection claims might not have gone through a fair refugee status determination procedure and if this is the case, then authorities may be putting at risk already vulnerable people," Tan said.

The Australian navy routinely turns back boats carrying asylum seekers from Africa, the Middle East and South Asia. However, these boats are usually returned to Indonesia, where most of the sea voyages begin.

Immigration Minister Peter Dutton has refused to confirm that the Vietnamese asylum seekers had been turned back, citing a policy of secrecy surrounding the interception of people-smuggling boats attempting to reach Australian shores.

His office issued a statement Tuesday saying that Australia had not beached its obligations under the United Nations Refugee Convention by returning refugees to the country where they risk persecution.

"The Australian government acts in accordance with its international obligations," the statement said.

Local media reported that the boat left Vietnam in March and was intercepted by Australian customs and navy vessels early in April.

In a rare update on its ocean surveillance of such boats, Dutton revealed in January that 15 asylum boats containing 429 asylum seekers had been turned back since his government was elected in September 2013.