Religious freedom is "constantly and brutally under siege" around the world, and the US must act decisively to end it, an influential Congressman has said.

Writing an op-ed in The Hill, Congressman Chris Smith, who is a key campaigner for religious freedom in the House, condemned restrictions on religious liberty.

Terrorist groups such as ISIS in the Middle East are "attempting to systematically exterminate ancient religious communities," he said, also highlighting violence and discrimination against minority faith groups in Burma, Pakistan, China and North Korea.

"In a world where some people are willing to kill and die for their faith...it is more important than ever for the US to engage in robust religious-freedom diplomacy," he said.

"Religious freedom can act to undermine the religion-related violence perpetuated by nonstate actors."

Smith earlier this year introduced the Frank Wolf International Religious Freedom Act of 2015, which calls for enhanced diplomacy, training, counter-terrorism and foreign assistance efforts. It would expand the role of David Saperstein, the ambassador-at-large for international religious freedom, which Smith says is a vital part of US foreign policy.

"The bill will elevate his office, sending the signal inside the government bureaucracy that this policy is a priority. More importantly, it will demonstrate to victims of religious persecution that they are not forgotten," he wrote.

"Diplomacy by itself will not stop terrorism, but coordinated with military and economic development efforts, it can help undermine the conditions that nurture groups such as ISIS and Boko Haram, as well as extremist Hinduism or radical Buddhism."

The bill has been supported by the US Conference of Catholic Bishops, as well as the ecumenical International Religious Freedom Roundtable.

In a letter to Congress, the Roundtable said that though "there is very little we agree on theologically, or politically", they were unanimous in agreement that the implementation of Smith's bill would "send a clear and urgent message regarding the inherent dignity of every human being, as well as our common global security in the fight against religious persecution, extremism, and terrorism."

The US State Department's latest International Religious Freedom Report released last month found that 2014 saw some of the most "egregious abuses of religious freedom and other human rights".

Saperstein told reporters that restriction on religious liberty is a growing problem globally. "In far too may countries people face daunting, alarming, growing challenges on account of their beliefs," he said.

"In countries where once proud traditions of multi-faith cooperation, positive coexistence was the norm, we have witnessed growing numbers of religious minorities being driven out of their historic homelands...We dedicate our work to their struggle and continue to fight for a world in which every individual is free to live out the core of his or her conscience."