"Senator Clinton and Senator Obama met tonight and had a productive discussion about the important work that needs to be done to succeed in November," said a statement issued by the two campaigns.

Obama campaign spokesman Robert Gibbs told reporters on a flight to Chicago that the Illinois senator met with Clinton but he declined to disclose the location or details of what they discussed.

Obama had been scheduled to fly back to Chicago on Thursday evening after a rally in northern Virginia, but skipped the flight and slipped away from the reporters travelling with him in order to meet secretly with Clinton.

Gibbs denied media reports that the meeting took place at Clinton's home in Washington.

As Obama enjoyed his first campaign swing as the likely Democratic presidential nominee, some prominent supporters of Clinton launched an effort to pressure him to invite her to join his ticket as the No. 2 in the general election battle against Republican John McCain.

But Clinton distanced herself from the push and said the decision on a vice president was his alone to make.

Critics of Clinton have accused her of trying to force her way on to the ticket. An aide to the New York senator issued a statement trying to dispel that impression.

"While Senator Clinton has made clear throughout this process that she will do whatever she can to elect a Democrat to the White House, she is not seeking the vice presidency, and no one speaks for her but her," said spokesman Phil Singer. "The choice here is Senator Obama's and his alone."

Backers of an Obama-Clinton ticket believe it would be the best way to unify the Democratic Party after the hard-fought, 16-month race between the candidates.

Obama made history on Tuesday when he became the first black to win a U.S. major-party presidential nomination. Clinton would have been the first woman to do so.

The former first lady did not immediately concede the race but told supporters in a letter on Wednesday she would hold an event on Saturday where she would formally back Obama.

Obama has not tipped his hand about whom he might pick as his running mate and when asked publicly about the option of choosing Clinton, he has praised her but emphasized his selection process would be deliberative and wide-ranging.

Clinton was seen as having promoted the idea of her becoming the vice presidential nominee when she told supporters in a conference call on Tuesday that she would be "open" to it if it would help her party win the White House.

Obama told reporters he appreciated the statement from Clinton's aide deferring to him on the running mate choice.

POTENTIAL VICE PRESIDENTIAL PICK

At the northern Virginia rally attended by 10,000 people, Obama shared the media spotlight with someone cited frequently by pundits as a potential running mate: Virginia Sen. Jim Webb.

Webb, who had remained neutral as Obama and Clinton battled for the nomination, gave the Illinois senator an emphatic endorsement as he introduced him.

"I'm honoured to stand alongside this man, a man of great intellect who over the past 16 months has impressed all of us as he stood up to sometimes withering attacks with measured responses, unshakable composure," Webb said.

The decorated Marine veteran of the Vietnam War said Obama "has given all of us confidence in the steadiness that we want to see in a commander in chief."

In his Virginia speech, Obama said he hoped he and McCain could have a respectful debate about policy issues and keep the campaign from getting bogged down by "name-calling" and "scandal-mongering."

The Illinois senator told McCain of that wish when the presumptive Republican nominee called Obama to congratulate him on Wednesday.

"I said that I was looking forward to a civil, substantive debate on the issues. And he agreed," Obama said, adding they discussed McCain's idea of appearing jointly at town-hall style forums. Obama's campaign has said it is open to such formats and the two camps are exchanging views on options.

But Obama did not hold back from attacking McCain. At an event in southwestern Virginia earlier in the day, Obama likened his Republican rival's health care proposals to those of the unpopular President George W. Bush. He said McCain's ideas amounted to "Bush light."

McCain's campaign hit back, deriding Obama's attempts to cast himself as someone who could rise above party divisions.

"Barack Obama has no record of bipartisan success," said McCain spokesman Tucker Bounds, adding Obama had voted "in lock-step with his party on issues from tax relief to funding of the Iraq war.