The Washington Post, citing unidentified Republican sources, said Powell had described John Bolton as 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.
The Senate Foreign Relations Committee agreed Friday to hold a vote on Bolton's nomination on May 12. The GOP-led panel will examine roughly a dozen allegations of abusive or high-handed behavior by Bolton, with an agreement to complete that work by May 6, said Andy Fisher, a spokesman for committee chairman Lugar.
There are no plans to ask Bolton to return for more questions.
Lugar was forced to postpone a vote scheduled for last Tuesday when it became clear that he had lost a crucial GOP vote.