Washington  - Former British prime minister Tony Blair on Thursday warned US President Barack Obama of the dangers of the public eye, telling him to rely on his faith and friendships when times get tough.

"Should it ever be tested, I hope your faith can sustain you and your family. The public eye is not always the most congenial," Blair told Obama during his keynote speech at the National Prayer Breakfast in Washington.

The annual breakfast brought into focus the links between religion and politics, as Obama addressed attendees before unveiling his office of faith-based programmes later Thursday.

Blair, who served as prime minister for 10 years, has drawn more attention to his personal religious beliefs than is typical in Britain and converted to Catholicism after leaving office.

He warned that religion was under attack from extremists within and non-believers without. "The extreme believers and aggressive non- believers come together in an unholy alliance," he said.

He also exhorted the new president to keep friends nearby. "You don't need cheerleaders, but partners, not spectators, but supporters. The truest friends are those still around when the going gets toughest."

The former prime minister also joked about Obama's coup in keeping his smartphone, overcoming concerns about presidential records laws and security breaches.

"Great news about the BlackBerry," he said, noting that he only received his first mobile phone after leaving office. Blair joked that he then tried to text a friend, but the response was "Sorry, who are you?", illustrating how quickly a former leader is forgotten. (dpa)