NEW YORK (CNN/Money) - Battered shares of United Airlines parent UAL Corp. got a lift Monday as a word of a new union vote raised hopes it could yet avoid bankruptcy, but the airline's weak financial condition led it to announce after hours it had missed nearly $1 billion in scheduled debt payments.

United mechanics, seen voting on a concession package last week, will vote on essentially the same offer they rejected in a new vote set for Thursday.

The world's No. 2 airline issued a statement after the market close that it had not made a $375 million debt payment on bonds backed by aircraft, as well as $500 million in payments to Kreditanstalt fur Wiederaufbau (KfW) and $45 million in other unspecified payments.

The company has a 10-business-day grace period to make most of the payments, although the grace period goes to only Dec. 9 for some of the money owed to KfW, a German state-owned development bank.

Few had expected it to make the scheduled payment Monday, especially with continued uncertainty about union concessions and the strong possibility of a bankruptcy filing looming. The company said it expects to make the payments within the grace periods if it gets approval of its application for $1.8 billion in federal loan guarantees. But time is running out for that approval, even with its mechanics scheduled to hold a new vote on concessions.

Shares of UAL (UAL: Research, Estimates), which plunged Friday after the union rejection, regained some of that lost ground in Monday trading, gaining 83 cents, or 34 percent, to close trading at $3.28. The rebound came after the company and International Association of Machinists announced a new vote Thursday on the $700 million pay cut backed by union leadership but opposed by 57 percent of members.

In a news release issued early Monday, District 141-M of the International Association of Machinists (IAM) said the company had agreed to revisions to address its 13,000 rank and file members' concerns about "quality of work-life issues" and determining which vacation days will be unpaid in the 2004-2007 period. The terms of the pay cut remain the same, the union said.

"Following the rejection of the tentative agreement, an overwhelming majority of the membership we spoke with expressed ongoing problems with quality of work-life issues and the inability to select which vacation days will be unpaid as the primary reasons for refusing the tentative agreement," said District 141-M President Scotty Ford in the release.

Union leadership had supported the rejected labor pact, and Ford again urged members to vote for the wage concession package.

"The District 141-M Executive Board strongly recommends ratification, as it is the final opportunity to avoid bankruptcy and protect against the elimination of our entire collective bargaining agreement," he said in a message to members on the union's Web site.

About 24,000 other IAM members at United, including other ground workers such as baggage handlers and customer service representatives, voted in favor of a similar pay cut package. Other union members, including pilots and flight attendants, have also ratified pay cuts. But none of the pay cuts will take effect without all the union members agreeing to the cuts.

The union concessions, which come to an estimated $5.2 billion over the next 5-1/2 years, are seen as essential if the airline is to win approval of $1.8 billion in federal loan guarantees it is seeking. The Air Transportation Stabilization Board (ATSB), the three-member federal panel considering the guarantee, has been meeting with United management and unions and has demanded the concessions if it is to consider granting the guarantees. But approval is not certain even if the mechanics reverse course and approve the tentative labor pact.

The new vote by mechanics Thursday will be the second time this year mechanics had been asked to vote again on essentially the same contract they had rejected. In February, 68 percent of IAM members at United voted against a contract offer even though it gave them an average 37 percent pay raise, partly because the pact had been proposed by government mediators rather than reached in collective bargaining. Three weeks later 59 percent of members voted for essentially the same offer.

CNNfn's Lisa Leiter reports on United's troubles and Thursday's upcoming vote. Play video (QuickTime, Real or Windows Media)

While the new vote lifted shares Monday, it did little to encourage analysts, who said that United will have a tough time avoiding bankruptcy even if it wins the rank and file vote and the federal loan guarantees, due to its continued losses.

"I don't there's enough reason today to buy the stock, given the horrible straights the company finds itself in," said Jim Corridore, airline equity analyst for Standard & Poor's. "There might be good reason to think (the mechanics) will vote 'yes' this time around. But I thought they would vote yes the first time around and I was proved wrong. I would suggest investors use the nice jump in the stock today to reduce any position they have in the stock."

Corridore said he wasn't surprised United had not made the loan payments due Monday.

"It would be reckless on management's part to pay it today," he said. He said that if the ATSB approves the loan guarantees before the end of the grace period Dec. 16, the airline will have the ability to make the payment, even if it hasn't closed on the new guaranteed loan. But he said it's also possible the airline will file for bankruptcy protection even with the loan guarantees and labor concessions as a way of reducing what it owes creditors.

"It would be very telling, whether or not they make the payment," he said. "If they decide to pay it, it's a sign they're going to do everything they can to stay out of bankruptcy."