A European Central Bank program credited with helping calm the continent's debt crisis is facing a legal challenge in Germany's top court.

Germany's Federal Constitutional Court will hear arguments Tuesday against the ECB's offer to buy government bonds, a policy that has helped lower borrowing costs for indebted countries.

Opponents of the purchases, known as Outright Monetary Transactions, or OMTs, say the program goes beyond the bank's mandate enshrined in EU treaties that forbid it from financing governments.

The ECB hasn't bought any bonds. But the program's mere existence has helped lower bond market borrowing costs and eased fears the 17-country euro currency union might break up.

The court hears arguments over the coming two days but a decision might not come for months.