Obama's encounter with Karzai at Mandela memorial just a greeting: White House --- (Reuters) - President Barack Obama's encounter with Afghanistan President Hamid Karzai at Nelson Mandela's memorial in South Africa was "just an exchange of greetings," a White House aide said on Tuesday. -- White House deputy national security adviser Ben Rhodes told reporters aboard Air Force One during the return flight to the United States that Obama had not seen the venue as a place to do business with Karzai, who has rebuffed what he calls pressure from Washington to accept a new security agreement.