Rasmussen's visit "underscores the vital importance the United States places on NATO as the cornerstone of our alliance with Europe and the importance of Rasmussen's leadership at this critical time", Xinhua quoted White House spokesperson Josh Earnest telling a press briefing.
On Ukraine, Obama and Rasmussen discussed "the implications of Russia's aggression for European security" and the efforts the US and other allies have been making over the past several months to reassure all allies of NATO's commitment to collective defence, Earnest said.
The leaders spoke about the need to improve allied defence investment and to bolster the defence capacity of NATO's vast network of partners, the official said.
Obama and Rasmussen also spoke about Afghanistan and planning for NATO's non-combat mission beyond 2014, he added.
After the meeting, Rasmussen told reporters that Afghanistan must sign a security agreement before the NATO summit in Wales in September, or there will be severe problems after 2014.
He also urged contenders in Afghanistan's presidential election to work with electoral authorities "to find a solution that ensures a credible outcome" of the election.