The head of the World Council of Churches on Tuesday joined the Roman Catholic archbishop of Boston in condemning the bombing at the Boston Marathon and offered prayers and support against violence.

The blast during the marathon, one of the world's most celebrated long-distance races, killed three, including a boy aged eight, and left more than 100 injured.

"This violence in the midst of what was to be a time of celebration and personal accomplishment as many from around the world gathered for a peaceful competition has brought pain and fear to so many across your country," said WCC general secretary, Rev. Olav Fykse Tveit in a statement.

The Federal Bureau of Investigation in the United States were still unsure of those responsible Tuesday, was treating it as an incident of terrorism.

Tveit sent a letter to the National Council of the Churches of Christ in the USA, addressed to the NCCCUSA transitional general secretary, Peg Birk and president, Kathryn Lohre.

"In this time when the sanctity of life must be proclaimed most strongly, I offer my personal support to your ongoing advocacy against violence in all its forms," Tveit said.

"In the name of the God of Life all of us must offer such witness as we are called to be agents of justice and peace in a too often wounded world."

In a message from Pope Francis, the Archbishop of Boston, Cardinal Sean O'Malley said in a statement Monday, "The Archdiocese of Boston joins all people of good will in expressing deep sorrow following the senseless acts of violence perpetrated at the Boston Marathon today.

"Our prayers and concern are with so many who experienced the trauma of these acts, most especially the loved ones of those who lives were lost and those who were injured, and the injured themselves.

O'Malley praised the citizens of Boston and state Massachusetts for their bravery and the heroism of many, particularly the men and women of the police and fire departments and emergency services.

"We stand in solidarity with our ecumenical and interfaith colleagues in the commitment to witness the greater power of good in our society and to work together for healing," said Cardinal O'Malley.