Christian Solidarity Worldwide is concerned that a new decree in Vietnam will seriously curtail religious freedom.
It describes Decree 92 as "vague" and "restrictive".
The decree was issued in November and came into force on 1 January.
It sets out the conditions for registration of religious activities and practitioners, and places new conditions on "foreigners" organising religious conferences and events.
Nguyen Hung of AsiaNews believes the decree follows the Chinese model of control of religions.
Nguyen's report cites Catholic priests and parishioners who believe the decree is a sign that the government is "looking for a way to control and restrict the freedom of religion".
The International Buddhist Information Bureau fears that the decree will increase state control of religions.
The 84-year-old leader of the Unified Buddhist Church of Vietnam, Thich Quang Do, who is under house arrest, also raised concerns in an interview with Radio Free Asia, saying that the new decree will "seriously curtail religious freedom" in Vietnam.
CSW's Advocacy Director, Andrew Johnston, said: "Whilst welcoming the Vietnamese government's recognition of the need for new guidance on the implementation of laws relating to religion in Vietnam, CSW is deeply concerned about the restrictive nature of Decree 92."
He said the decree appeared to restrict religious activities in a manner that was inconsistent with Vietnam's obligations under the International Covenant on Civil and Political Rights (ICCPR), particularly Article 18 which relates to freedom of religion or belief.
Mr Johnston added: "CSW urges the Government of Vietnam to ensure that the right to freedom of religion or belief is fully guaranteed by law."