Bishop Angaelos has welcomed the US' declaration that ISIS has committed genocide against Christians, Yazidis and Shia Muslims.

For only the second time in history, the US administration today declared an ongoing conflict to be genocide. The term is understood to signal the gravest possible crime against humanity.

"In my judgment Daesh is responsible for genocide against groups ... under its control, including Yazidis, Christians and Shia (Shi'ite) Muslims," said US secretary of state John Kerry, using an Arabic derogatory term for ISIS.

He added the militant group was genocidal "by self-proclomation, by ideology and by actions".

He went to say the US did not have the full picture but would support efforts to gather further evidence.

"The fact is that Daesh kills Christians because they are Christians, Yazidis because they are Yazidis, Shia because they are Shia," Kerry said.

He continued: "Naming these crimes is important, but what is essential is to stop them."

In an interview with Christian Today, the general bishop of the Coptic Church in the UK, Bishop Angaelos, said he had not expected the judgement.

"I was in the States last week and there seemed to be such reservation about declaring a genocide against Christians that the general expectation was it would be genocide for other groups but not Christians," he said.

"But it is very welcome that now both Congress and the [Obama] administration have declared a genocide against Christians as well as Yazidis and Shia Muslims."

He said that his concern until this point had been there would be "partial recognition" for either a Christian or a Yazidi genocide. He said that "would have left the unrecognised group at risk".

A declaration of genocide places obligations on the US to intervene. The US' deliberations over whether to use the term echoed Clinton's failure in 1994 to declare the Rwandan conflict a genocide, which he later regretted.

The announcement made by Kerry on Thursday followed a unanimous vote in the House of Representatives on Monday to use the term. It also comes after the European Parliament declared the atrocities committed by ISIS a genocide in February.

Bishop Angaelos has become heavily involved in the response to the conflict as well the ensuing refugee crisis as many Christians caught up in the violence are Copts. This became particularly apparent after a video was released by ISIS depicting the murder of 21 Copts on a beach in Libya in February 2015.

However, Angaelos said: "This was never really just about the Coptic community. This was about speaking for all groups who have been subject to horrific treatment."

Angaelos said what followed now was a matter for international law. He added he would like support for communities currently at risk to be top of the global leaders' priorities after this declaration.

"I hope the implication of this declaration for people under ISIS' rule is they feel we understand their suffering and recognise what have been going through," he said.

"I hope they will recognise we will stand with them going forward."