WASHINGTON, March 25 (Reuters) - US President Joe Biden on Thursday (Mar 25) said he would prevent China from passing the United States to become the most powerful country in the world, vowing to invest heavily to ensure America prevails in the rivalry between the world's two largest economies.

Biden said he had spent "hours upon hours" with Xi Jinping when he served as vice president under former President Barack Obama, and was convinced the Chinese president believed autocracy - not democracy - held the key to the future.

The Democratic president said he had made it clear to Xi that the United States was not looking for confrontation, but would insist China abide by international rules for fair competition, fair trade and respect for human rights.

"China has an overall goal ... to become the leading country in the world, the wealthiest country in the world and the most powerful country in the world," he told reporters at the White House. "That's not going to happen on my watch because the United States is going to continue to grow."

Biden took aim at Xi and Russian President Vladimir Putin for embracing autocracy.

"He's one of the guys, like Putin, who thinks that autocracy is the wave of the future, (and) democracy can't function in an ever-complex world," the president told his first news conference since taking office in January.

Earlier in March, Biden told ABC News he believed Putin was "a killer", which sparked fury in Moscow.

"He (Xi) doesn't have a democratic - with a small 'd' - bone in his body, but he's a smart, smart guy," he said.

Biden's remarks echoed those made during his presidential campaign, and come days after the first high-level, in-person talks between US and Chinese officials under his administration in Alaska, where combative public statements laid bare the depth of tensions between the rivals.

In an interview with CNN's Christiane Amanpour, the Chinese ambassador to the United States, Cui Tiankai, said China's goal is to "meet the growing aspiration of the Chinese people for a better life". "Our goal is not to compete with or replace any other country. This is never our national strategy."