Sinn Féin leader Mary Lou McDonald has said Britain's Northern Ireland Secretary of State, Karen Bradley, should apologise over comments in London's House of Commons.

Ms Bradley earlier claimed that killings carried out by security forces during the Troubles in the North "were not crimes".

She said they were simply people carrying out their duties "in a dignified and appropriate way."

Ms Bradley was responding to a question from a DUP MP.

Emma Little-Pengelly asked Ms Bradley when mechanisms were going to be put in place to investigate murders carried out during the Troubles.

Ms Pengelly said: "Well over 90% of the murders and injuries caused during the Troubles in Northern Ireland were caused by acts of terrorism.

"Very few prosecutions and investigations are underway, innocent victims are being left behind with thousands of unsolved cases.

"When is the Secretary of State going to address this issue and put in place a mechanism to investigate the over 90% of acts of terrorism that cause those murders?".

Ms Bradley replied: "The honourable lady sets out the figures very, very powerfully.

"Over 90% of the killings during the Troubles were at the hands of terrorists. Every single one of those was a crime.

"The fewer than 10% that were at the hands of the military and police were not crimes.

"They were people acting under orders and under instruction and fulfilling their duties in a dignified and appropriate way."

"I look forward to working with her more to ensure that we can deliver the much needed reforms and changes that we all want to see".

Mary Lou McDonald speaking to reporters outside Leinster House | Image: Michelle Devane/PA Wire/PA Images

Ms McDonald said she has written to the British Prime Minister Theresa May and Taoiseach Leo Varadkar following the comments.

She has called on Ms Bradley to withdraw her comments and apologise to the families who have lost loved ones at the hands of British forces.

"Karen Bradley's comments have caused great distress to the many victims of British state forces.

"In my letter I reminded Prime Minister May that these families have been denied Article Two compliant investigations, inquests have been stalled and information withheld.

"They have been forced to take court cases to vindicate their basic rights and still face ongoing cover ups.

"The British Secretary of State should immediately withdraw her comments and apologise to these families.

"These comments can only be construed as political interference in the judicial system which is unacceptable."

Sinn Féin's deputy leader Michelle O'Neill has branded the comments as "outrageous and offensive".

Ms Bradley has previously said she did not understand politics in the North when she started the job.

Last year, she admitted she did not understand "some of the deep-seated and deep-rooted issues that there are in Northern Ireland".

She explained: "I didn't understand things like when elections are fought for example in Northern Ireland, people who are nationalists don't vote for unionist parties and vice-versa. So, the parties fight for the election within their own community.

"Actually, the unionist parties fight the elections against each other in unionist communities and nationalists in nationalist communities. That's a very different world from the world I came from."