The Washington Post, citing unidentified Republican sources, said Powell had described John Bolton an an intelligent but problematic government official in conversations with Sen. Lincoln D. Chafee, R-R.I., and Sen. Chuck Hagel, R-Nebraska.
Both men are members of the Senate Foreign Relations Committee, the panel that is weighing Bolton's confirmation.
Separately, the New York Times reported that Powell had raised concerns about Bolton's harsh treatment of subordinates while he was a State Department official.
Both newspapers said Powell's warnings about Bolton had been mixed with praise for his performance on some matters.
Powell was the only former Republican secretary of state who did not sign a letter of support for Bolton that was sent to the committee chairman, Sen. Richard Lugar, R-Ind.
Despite growing problems with the nomination, the White House has remained firm in its backing of Bolton.
"He is the right man at the right time for this important assignment. I urge the Senate to put aside politics and confirm John Bolton to the United Nations," President Bush said Thursday.
Mr. Bush blamed partisan bickering in the nation's capital — "where sometimes politics gets in the way of doing the people's business" — for the difficulties Bolton's nomination has encountered.
Those difficulties increased on Thursday as one of the president's former ambassadors told CBS News Correspondent Gloria Borger that Bolton had been less than truthful in his recent confirmation hearings.
The episode revolves around a speech Bolton gave in South Korea in the summer of 2003, in which he said, "For many in North Korea, life is a hellish nightmare."
When asked about the hard-line speech, Bolton said Ambassador Thomas Hubbard had approved it.
"I can tell you what our ambassador to South Korea, Tom Hubbard, said after the speech. He said, "Thanks a lot for that speech, John. It'll help us a lot out here.'"
That's not what Hubbard says. In fact, the ambassador told CBS News that he specifically objected to the tone of the speech and actually found it unhelpful in dealing with North Korea.
"I told the committee that if you're basing your vote on his (Bolton's) assertion that I approved his speech, that is not true," Hubbard said.
And there's more. Hubbard also told CBS News about Bolton's explosive temper, recalling how Bolton slammed the phone down after learning he couldn't get a meeting he wanted. Then Bolton backed out of a dinner in his honor that night.
"The temperament was on display," the ambassador said.
Committee chairman Lugar is planning on a vote the week of May 9. He is trying to work out with the committee's senior Democrat, Joseph Biden of Delaware, whether to hold more hearings and take more statements from supporters and opponents of Bolton in the interim.
Under consideration is summoning Bolton to testify again.
"We think that the issues have been addressed," State Department spokesman Adam Ereli said. "We think we've answered the questions. We would like things to move forward."
But, Ereli said, "if the committee or chairman request or call for additional testimony we will work with the committee."