WASHINGTON -- US officials on Tuesday refused to allow the man chosen by Illinois's scandal-tainted governor to replace president-elect Barack Obama to be sworn in at start of a new Senate session.

"I presented my credentials to the secretary of the senate, and advised that my credentials were not in order," said Roland Burris, surrounded by his lawyers, before a media scrum in freezing rain outside the Capitol building.

"I am not seeking to have any type of confrontation. I will now consult with my attorneys, and we will determine what our next step will be," Burris added.

He was picked last week to fill Obama's vacant Senate seat by Illinois governor Rod Blagojevich, who was arrested in December for alleged corruption, including seeking to sell the empty Senate seat to the highest bidder.

Obama, who was the only African-American senator and resigned his seat after the winning the November 4 presidential elections, has said he regrets Burris's appointment and that the governor should instead resign.

Lawyer Timothy Wright said Burris now had several options including further negotiations with top Senate lawmakers or seeking to win a court ruling.

He said Burris had not been allowed to go onto the floor of the 100-seat Senate or to take the oath of office.

"All of which we think was improperly done. And it is against the law of this land. We will consider our options," Wright said.

Burris will remain in Washington until he can "deliberate with the Senate leadership and perhaps get them to reverse themselves," said Wright, adding that there could be a possible court challenge to the decision.

Burris' appointment has not been signed by the Illinois Secretary of State, as required by Senate rules, and has been rejected by Senate Secretary Nancy Erickson, her spokeswoman said.

But Burris's attorneys were apparently unconcerned about the legal uncertainty.

Citing a "200, 300-year-old case" as precedent, Wright told reporters: "From that we have determined that senator Roland Burris is the junior senator from the state of Illinois."

Senate Majority Leader Harry Reid has opposed Burris's nomination due to the corruption scandal.

But Burris maintains the scandal surrounding Blagojevich "is all politics and theater."

CNN reported that a deal could be in the making between Democratic lawmakers and Burris that would have the Illinois statesman take his Senate seat as long as he vows not to run for in the 2010 legislative elections, when the seat comes up for grabs.

Burris has also been asked by the Illinois House of Representatives committee investigating whether to impeach Blagojevich to appear Wednesday in the state capital Springfield.

New senators elected in the November elections as well as the new House of Representatives were being sworn in Tuesday to take up their seats in the 111th Congress.