US President Barack Obama said Tuesday the first presidential debate left him worried about the positions Donald Trump has espoused on nuclear weapons and climate change.

Watching the showdown from the White House Treaty Room, Obama said he enjoyed a "pretty vigorous debate about where we need to take our country" in between reading policy memos and to-do lists.

The debate, Obama said, showed Trump "doesn't have the preparation, the temperament or the core values of inclusion and making everybody have opportunity that would take our country forward."

Phoning into "On Air with Ryan Seacrest," Obama -- a frequent critic of the Republican nominee -- said Trump's positions should spur younger Americans to register to vote. He called this year's contest "especially big" because of the "big differences between the two candidates."

He praised Hillary Clinton's performance, saying the Democrat was "well prepared, she's got the right temperament for the job, she's well-respected around the world, she's serious, she does her homework."

And he expressed deep concerns about Trump's readiness for the job, telling Seacrest the GOP nominee falls short on two issues he's concerned about most for his daughters' generation.

"I get worried when I hear somebody like Donald Trump start saying, 'Well, I don't necessarily know whether Japan or Korea should be protected by us,' Obama said. "Maybe they should get their own nuclear weapons. That shows somebody who doesn't pay attention to these issues, and you don't necessarily want close to the nuclear button."

He hit Trump for tweeting that climate change is a hoax perpetrated by China, a line that Clinton utilized Monday night.

"When you hear somebody like Trump say he thinks this is a plot by the Chinese, that this is a fraud and a hoax, when 99% of scientists are saying you've got to do something about it, that worries me," he said.

Agencies