US Treasurys are extending a sell-off this week on Wednesday alongside major bond markets in Europe and Asia.

Some investors are speculating that tax cuts could stoke economic growth and alter how the Fed approaches interest-rate increases.

There's also concern that the Fed's reduction of its bond purchases would increase the market's supply.

The 30-year bond had its worst sell-off in a year on Tuesday, while the 10-year yield has climbed to its highest level in nine months.

US Treasurys are slipping on Wednesday ahead of the House's second vote on the GOP tax bill.

Bonds markets across Europe and Asia are also selling-off, sending yields higher.

The House passed the Tax Cuts and Jobs Act on Tuesday, but needs to vote again Wednesday after three pieces of the bill were deemed in violation of the so-called Byrd rule. It would be an identical version of the bill the Senate passed just after midnight on Wednesday.

As the House version passed on Tuesday, the Treasury yield curve bear-steepened, meaning that long-term rates rose faster than short-term rates as traders speculated on quicker growth. On Tuesday, the 30-year yield had its biggest increase in a year. The benchmark 10-year yield rose to its highest level in nine months, and has climbed by about 14 basis points since December 15.

Here's the scoreboard of Treasurys as of 8:51 a.m. ET:

2-year +

5-year +0.2 bps at 2.235%

+0.2 bps at 2.235% 10-year +0.2 bps at 2.486%

+0.2 bps at 2.486% 30-year +0.3 bps at 2.855%

According to Bloomberg, this sell-off is also happening as investors unwind trades that had bet the yield curve would flatten. There's also concern that the Fed's reduction of its bond purchases would increase the market's supply, putting more downward pressure on prices and raising yields.

The difference between between longer- and shorter-dated Treasury yields, such as 2- and 10-year yields, recently fell to their lowest level in a decade, prompting a debate across Wall Street about whether it's signaling, this time around, that the economy is about to slow down.

With the Fed now raising interest rates, Treasury yields are becoming more alluring to bond-market investors as a source of income. Notably, the two-year yield has now climbed to match the S&P 500 dividend yield for the first time since the 2008 financial crisis, Bloomberg data shows.

Federal Reserve Chair Janet Yellen said last week that the central bank's most recent economic forecasts should not be seen as estimates of the impact of tax cuts.