As U.S. Sen. Bernie Sanders prepared to board a plane for New Hampshire, he told the Des Moines Register that he is "extremely disappointed" that the results of Monday's Iowa caucuses have not been released.

"Obviously, I suspect I speak for all of the candidates, I am extremely disappointed by the inability of the Iowa Democratic Party to come up with the results in a timely manner. I don’t know why in 2020 it should take so much time," Sanders said in a car at the Des Moines airport tarmac.

Iowa Democratic Party officials have said they would release partial results from Monday's caucuses at 4 p.m. CST, amid critiques and confusion over the lack of any results data Monday night.

Jeff Weaver, a Sanders' senior adviser, told the Register that with internal results from about 60% of the state, Sanders was leading in first alignment popular votes, followed by former South Bend, Indiana, Mayor Pete Buttigieg and U.S. Sen. Elizabeth Warren.

"That’s a pretty representative sample," Weaver said.

Before he left for the first-in-the-nation primary state, Sanders thanked his Iowa volunteers.

"I want to thank, once again, thousands of Iowa volunteers that we had who were part of an extraordinary grassroots movement which knocked on hundreds of thousands of doors which enabled us to do as well as we did," the Vermont senator said. 

“On to New Hampshire!” he concluded.