Story highlights Obama has been a frequent critic of Donald Trump

Obama watched the debate from the Treaty Room in the White House

Washington (CNN) 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."

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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."

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