Troubled U.S. trucker YRC Worldwide (YRCW.O) said it was again extending a crucial debt-for-equity offer as it continued to work to convince bondholders to participate.

The company said early Tuesday 81 percent of the company's outstanding notes totaling $536.8 million had been tendered for an exchange for equity. That is up from about 80 percent a week ago.

The company had said the offer would expire late Monday night but said Tuesday it would extend the offer until 11:59 p.m. EST on December 29, the fifth such deadline set for the exchange offer over the last few weeks.

YRC, which is the largest U.S. trucking firm handling smaller, or less-than-truckload shipments, is struggling to stay out of bankruptcy and has hinged its hopes on a financial restructuring based on the debt-for-equity swap.

On Tuesday, YRC said 92 percent of the aggregate principal amount of its 5.0 percent and 3.375 percent notes and 53 percent of the 8-1/2 percent notes had been tendered into the exchange offers, representing 81 percent of the company's outstanding notes.

But the company has said it must have 70 percent of the aggregate principal amount outstanding of the company's 8-1/2 percent notes, along with at least 85 percent of the aggregate principal amount outstanding of the 3.375 percent notes and the 5 percent notes on a combined basis for the exchange to be successful.

The company said it was continuing to work to increase levels of support.

The YRC statement offered both bad news and good news, said Wall Street Strategies analyst David Silver.

It's bad in the fact that they aren't getting the needed people to step up and convert their shares, said Silver. But they keep on inching higher. Eventually I think they'll get there.

Silver said he expected YRC to sweeten the deal for the holders of the 8-1/2 percent notes with some additional incentives.

You're going to see YRC really step up and say this needs to get done, he said.

(Reporting by Carey Gillam, editing by Dave Zimmerman)