The president should not be making such claims without evidence to back it up. Period!
That being said, I wonder if the NSA sought and received warrants to hack the cell phones of the German Chancellor or Mexican President?  Or listen in on calls made to and from the Kremlin?
Everyone knows the government has the ability to do these things . I dont find it too far fetched to believe they would do it to American citizens as well, with or without a warrant under the right circumstances. 
Snowden confirmed as much in his illegal release of NSA documents.