Dozens of Christians are currently being held by the authorities in Iran and many of them because of their involvement in house churches, the UN reports.
The warning came from the United Nations Special Rapporteur on the Situation of Human Rights in the Islamic Republic of Iran, Ahmed Shaheed, in a new 28-page report due to be presented at the meeting of a General Assembly human rights committee on Tuesday.
Shaheed said there were at least 300 people currently detained in Iran because of their religious practices, among them 49 Christians and 120 members of the Baha'i community.
The report states that pastors have been arrested for holding services in Persian or for "allegedly ministering to Iranians from Muslim backgrounds".
"At least 49 Protestant Christians are currently detained, many for involvement in informal house churches," the report says.
"In April 2014, security forces reportedly raided an Easter service in a private home in southern Tehran and detained six individuals."
Also suffering persecution are lawyers who have taken on sensitive Baha'i or Christian cases, with many of them being imprisoned or having to flee the country, the report notes.
There had been hopes that religious freedom would improve under President Hassan Rouhani after he promised before being elected in June 2013 that all religions "must feel justice".
However, Shaheed's report suggests little has changed.
One pastor still imprisoned for his faith in Iran is American-Iranian Saeed Abedini, who is serving an eight-year sentence for supposed crimes against Iranian national security. It is believed his imprisonment is largely to do with his involvement in the house church movement prior to moving to the US with his American wife in 2005.
On the two-year anniversary of his imprisonment last month, Christians around the world took part in a day of prayer for Abedini's release.
A letter to his 8-year-old daughter, Rebekkah Grace, was read out on the day in which he said: "You are growing so fast and becoming more beautiful every day. I praise God for his faithfulness to me every day as I watch from a distance through the prison walls and see pictures and hear stories of how you are growing both spiritually and physically."
Another pastor imprisoned on similar charges is Behnam Irani, who Jason DeMars of Present Truth Ministries recently asked Christians to pray for because of recurring health problems behind bars.