Rouhani -- in New York last month for the UNited Nations General Assembly -- spoke with U.S. President Barack Obama by telephone in an effort to ease tension over Iran's nuclear program, the BBC reported.
The conversation was the first between Iranian and U.S. leaders in more than 30 years.
"We support the diplomatic initiative of the government and attach importance to its activities in this trip," Khamenei said on his website.
However, Khamenei said "some of what happened on the New York trip was not appropriate."
"We are pessimistic towards the Americans and do not put any trust in them. The American government is untrustworthy, supercilious and unreasonable, and breaks its promises," Khamenei said without giving more detail.