The diplomat, described as a senior Western representative, met with reporters in Tel Aviv for a not-for-attribution briefing, Haaretz reported.
The United States hopes to convince Palestinian leaders that direct negotiations with Israel are the only way to achieve the goal of statehood, the diplomat said.
"If the P.A. will go to the U.N. in September, it will make it harder for us to have the same relations with them as we had before when it comes to aid and security training," the diplomat said. "We want that to continue that cooperation but it will make it harder for us. It is easier to work together as partners."
The diplomat also urged Israel to keep talking with the authority. Foreign Minister Avigdor Lieberman recently said Israel should cut off ties with the authority.
At a meeting Wednesday with 26 members of the U.S. House of Representatives, Israeli President Shimon Peres said U.N. recognition would be "meaningless."
"Peace is encountering difficulties, as always, but I believe it is achievable," Peres said.