US business tycoon Donald Trump has announced he is running for the White House, promising to "make America great again".

"I am officially running for president of the United States and we are going to make our country great again," he said in a speech delivered from Trump Tower on New York's Fifth Avenue.

"Look. I came down very hard on Mitt Romney because he let us down. He really let us down. That should have been an election that was won.

"That was an easy election to win. I actually think probably easier than the next one coming up. There was no excuse for not winning that..." he said.

The 69-year-old is considered a wildcard candidate in an increasingly crowded pool of contenders seeking the presidency in 2016.

Denouncing US foreign and economic policy, Mr Trump ridiculed America's current crop of politicians and vowed to reverse what he called US decline.

"Our country is in serious trouble. We don't have victories anymore," he said.

"When was the last time anyone saw us beating, let's say, China in a trade deal. They kill us. I beat China all the time."

"When did we beat Japan at anything? They send their cars over by the millions and what do we do?"

The businessman, whom Forbes values at $4.1bn, said "the United States has become a dumping ground for everybody else's problems."