United States presidential hopeful Senator Barack Obama. - Reuters

WASHINGTON (Reuters):

The United States' Secret Service yesterday began protecting U.S. Senator Barack Obama, a Democrat running for president, but would not comment on the reason why.

Homeland Security Secretary, Michael Chertoff, authorised the additional security protection for Obama of Illinois and "the Secret Service is now implementing that protection," said Eric Zahren, a Secret Service spokesman.

"We're not going to discuss... any of the deliberations or assessments that went into making that decision," he said.

Obama, who has served only two years in the U.S. Senate and eight years in the Illinois state Senate, is seeking to become the first black president in the history of the United States.