By Liana B. Baker

RIO DE JANEIRO (Reuters) - NBC, a unit of Comcast Corp, has sold $1.2 billion in advertising for the Summer Olympics in Rio de Janeiro, the network said on the eve of a rare Games where some of the most high-profile contests will air live in prime time for U.S. audiences.

The official U.S. broadcaster of the Games is selling ads at a quicker pace than the London Olympics in 2012, NBC Sports Group's executive vice president of advertising sales Seth Winter said on Thursday.

The problems surrounding the Rio Olympics have helped boost interest in the Games, Winter added. Instead of basking in the glory of hosting the first Olympics in South America, organizers were struggling to get the event ready.

Brazil has been grappling with a volatile political situation, concerns over crime levels in Rio, water pollution, late infrastructure and an outbreak of the Zika virus, which has clouded the start to the Games.

"It sounds a little bit distorted or perverse, whatever the appropriate word is, but it just raises awareness that there are Olympic Games going on in South America and you'd have to be hibernating under a rock not to understand that," Winter said, speaking from Rio on a conference call with reporters.

By nature of a short time difference between Brazil and the United States, the U.S. broadcast will feature more live events such as track and field and swimming than in London, over 17 consecutive nights from Aug. 5-21.

The live events give NBC "confidence in committing to our advertisers the types of numbers that we think we can reach," Winter said.

The broadcaster is still planning to air the opening ceremony on Friday on a tape delay, however, which has generated criticism on social media.

NBC signed on two years ago to pay $7.65 billion for the right to air six Olympic Games from 2022 to 2032 before any other U.S. media company could bid. Winter alluded to the Rio games being profitable to the network.

Networks have been stocking up on live sports, which are popular with advertisers since audiences cannot skip commercials.

More than 75 percent of the ad sales are driven by the main broadcast network, Winter said, although they also include cable and digital ad sales. NBC had previously said it hit the $1 billion ad sales mark for Rio four months ahead of the previous Summer Games.

(Reporting by Liana B. Baker in Rio De Janeiro; Editing by Meredith Mazzilli)