Germany's highest court is asking authorities to provide more evidence that they no longer have paid informants inside the country's main far-right party, signaling a potential hitch in a move to ban the group.

Parliament's upper house, which represents Germany's 16 states, in 2013 applied for a ban of the National Democratic Party. It alleges that the party promotes a racist, xenophobic and anti-Semitic agenda in violation of Germany's constitution.

In 2003, the Federal Constitutional Court rejected a previous attempt to ban the party because paid government informants within the group were partially responsible for evidence against it.

State governments say this application contains no information from informants. However, in a decision published Monday, the court demanded more evidence to back their assertion that they stopped using informants.