Procedures for the trial were discussed privately by Senators ahead of the official opening of the 106th Congress.

Senators are still undecided as to how to conduct the hearings into charges of perjury and obstruction of justice against the president.

Senate Republican leader Trent Lott said on Wednesday he could not predict how long the trial might take.

He said: "All kinds of variables are coming into play ... we may not know the absolute answer until we get started."

Peter Marshall reports on new attempts to reach a compromise between Republicans and Democrats

The Senate is not scheduled to formally debate impeachment issues.

In the House, one of the first orders of business is to reappoint the 13 House Republicans who will serve as prosecutors in the Senate trial.

The hearings are due to start at 1300 (1800GMT) on Thursday with the swearing in of Chief Justice William Rehnquist as presiding official, and of the 100 Senators as jurors.

So far there is no consensus on the format, scope or duration of the hearings. Most Democrats favour a short trial, with a vote possibly before the end of the month, but the vast majority of Republicans prefer a longer trial which would include calling witnesses.

Senate Democrat leader Tom Daschle said on Tuesday that starting the trial without a defined plan for proceeding would be an "invitation to a prolonged process that would take weeks if not months".

White House Spokesman Joe Lockhart said the uncertainty leaves the president's lawyers "preparing for all eventualities".

Senator Lott said the proceedings would follow the rules set down by the constitution, and produce a trial the American people would feel comfortable with.

The House of Representatives passed two articles of impeachment against Mr Clinton last month, charging him with lying under oath and obstruction of justice in the Monica Lewinsky affair. Click here for an explanation of the impeachment process.

Constitutional duty

"I'm going to do my best to make sure that we complete our constitutional duty under the rules of the Senate and come to a conclusion that is fair and hopefully quick, also," Mr Lott said.

The Senate leader is thought to favour a bipartisan proposal for a truncated trial that would test whether enough senators felt that the charges against the president were serious enough to merit his removal from office.

The votes of 67 of the 100 senators are needed for the conviction and removal from office of the president.

Trent Lott: We have a serious constitutional duty

The leaders of the faction calling for a quick way out say that a blocking minority against removing Mr Clinton already exists so there is no need to spend three months or so on a trial to reach a pre-ordained judgement.

But the proposal for a shortened trial has been fiercely opposed by Mr Lott's fellow Republican Senators and from Congressmen in the House of Representatives.

No compromise

Such a compromise, they claim, would trample over the constitution, and they have argued instead for a lengthy trial in which witnesses would be called.

The constitution does not say there has to be a full trial

A fierce Clinton critic, Senator James Inhofe, told the BBC's Newsnight: "It's not so much a case of Conservatives against Liberals, but that we have to have a trial because we want to uphold our responsibilities and our duty."

But leading constitutional lawyer Professor Mary Cheh emphasised that there was no constitutional requirement to have a trial.

"Those senators that are saying a full trial is constitutionally required really have no text to point to," she said.