"Thirty-one teams get their hearts broken," Howard, 35, said. "It has to end sometime. It ended a little bit early for us."

Before exiting, the US showed the spunk that won America's attention. The Belgians built a two-goal lead when Kevin De Bruyne scored in the 93rd minute and Romelu Lukaku in the 105th.

Then Julian Green, at 19 the third-youngest player in the tournament, volleyed in Michael Bradley's pass over the defence in the 107th, two minutes after entering. "I was sure that we would make the second goal and we would go to the penalty shoot-out," Green said.

The Americans nearly did. In the 114th, Clint Dempsey peeled away on a 30-metre free kick by Bradley, who passed ahead to Chris Wondolowski. He fed Dempsey, and goalkeeper Thibaut Courtois bolted off his line to block the close shot.

At the final whistle, the US players fell to the field in their all-white uniforms like so many crumpled tissues.

"They made their country proud with this performance and also with their entire performance in this World Cup," said Juergen Klinsmann, the former German World Cup champion who took over as coach three years ago.

Klinsmann called on his players to toughen their mental approach as he begins the long road towards the 2018 World Cup.

"We are still learning to take our game to the opponent," he said. "No matter what their name is, we cannot just wait too long to start our game.

"I think there is a little bit too much respect - why not play them eye to eye? I don't know how many years that takes to change but it's something we have to go through.

"We will find ways to introduce new young players and develop our programme to develop the game at every level," Klinsmann said. "We're excited about building a next Olympic team cycle which is huge for us going to Rio de Janeiro in two years."

The Americans advanced from a difficult first-round group to reach the knockout rounds of consecutive World Cups for the first time. Four years ago, they were eliminated by Ghana 2-1 on a goal in extra time.

"Getting to the round of 16, if we don't do that, we're very, very disappointed," US Soccer Federation president Sunil Gulati said. "We get here and it's kind of the swing game. We get beyond here, then it's generally viewed as very successful - this year was a little different because of the group we had in the first round, so that I think is already a success."

Fifa secretary general Jerome Valcke said: "What we have seen during this World Cup in the United States is incredible. Huge interest, record television figures, even surpassing the NBA."

More than 15 million Americans tuned in to watch Germany defeat the United States 1-0 in their group finale, broadcasters ESPN and Univision said.

Their 2-2 draw with Portugal pulled in an average 18.2 million viewers on ESPN alone to become what the network called "the most-viewed soccer match in the United States ever".

The crowd of 51,227 at Arena Fonte Nova appeared to be about one third pro-US. In the US, a huge crowd watched at Chicago's Soldier Field.

President Barack Obama joined about 200 staffers in an Executive Office Building auditorium to watch the second half.

"I believe!" he exclaimed as he walked in at the front of the hall. "I believe!"