Pope urges Bostonians 'to combat evil with good'

Pope Francis on Tuesday expressed his condolences over a twin bomb attack at the Boston Marathon as a "senseless tragedy" and urged the US city's residents "to combat evil with good", the Vatican said.

"The Holy Father prays that all Bostonians will be united in a resolve not to be overcome by evil, but to combat evil with good, working together to build an ever more just, free and secure society," the Vatican's Secretary of State said in a message to the archbishop of Boston, Sean O'Malley.

"Pope Francis wishes me to assure you of his sympathy and closeness in prayer. In the aftermath of this senseless tragedy, His Holiness invokes God's peace upon the dead, his consolation upon the suffering and his strength upon all those engaged in the continuing work of relief and response," Secretary of State Tarcisio Bertone said.

A massive probe was under way Tuesday after two bombs struck the Boston Marathon, killing at least three people and wounding more than 100.

Boston has a large Catholic population, mainly due to Irish, Italian and Polish immigration in the 19th and 20th centuries.

The archdiocese counts around 1.8 million baptised Catholics on its territory, making it the fourth largest archdiocese in the United States.