U.S. President Donald Trump is "frustrated" by the fact that the U.S. and Japan have yet to ink a trade deal, which was reflected in his recent mention of August as a desirable deadline, the U.S. top envoy to Japan has said.

"The president would have preferred to have seen something done last year. Let me be clear ... we're way behind schedule from his standpoint," U.S. Ambassador to Japan William Hagerty said in an exclusive interview with The Japan Times on Tuesday.

Tokyo and Washington kicked off an economic dialogue between U.S. Vice President Mike Pence and Deputy Prime Minister Taro Aso in April 2017 soon after Trump was sworn in. Negotiations on a possible free trade pact kicked off in April.