Commission head Jean-Claude Juncker also threatened Poland with suspending its voting rights in the European Union on Wednesday, a move which Hungary has already said it would veto.

Separately, the top European Union court's adviser dismissed a challenge brought by Slovakia and Hungary against the obligatory relocation of refugees across the EU, a plan also fiercely opposed by Poland and the Czech Republic.

The Czech crown, the forint and the zloty eased by 0.1 percent against the euro by 1227 GMT, and government bond yields rose by a few basis points.

But the moves were modest relative to bigger swings in the zloty over the past week due to worries over the Polish reform bills. These were partly vetoed by Poland's president Andrzej Duda, but the street protests they triggered and the risk of EU sanctions have added to investors' concerns.

Investors held their breath ahead of the Federal Reserve's meeting on Wednesday. Any reference to upcoming cuts in the Fed's monetary stimulus could lead to a weakening of currencies and bonds in Central Europe, traders said.

The region's healthy economic growth has made its assets relatively resilient to a shift towards a tighter monetary stance by the world's main central banks, even though they have not been immune to a rise in debt yields in the US and Germany.

Central banks in the region have not lifted their own interest rates since a hike in Belgrade in early 2013.

The closest to rate tightening is the Czech central bank (CNB), which some analysts have said may deliver its first hike as soon as at its next meeting, on Aug 3.

But investors are split over the timing, partly because many believe a firming of the crown will do the bank's work even without a hike.

The scope for tightening is limited by the European Central Bank's quantitative easing, CNB Vice Governor Vladimir Tomsik said on Wednesday.

In Poland, any rise in inflation risks could reignite central bank interest rate hike speculation and, along with healthy economic growth, could help the zloty, analysts said.

"While we therefore accordingly forecast a stronger PLN in our base scenario for the coming months, an escalation of the political uncertainties would require a revision of our PLN forecast," Raiffeisen analyst Wolfgang Ernst said in a note.