Two South African legal groups are challenging a decision by President Jacob Zuma to extend the term of the country's top judge.

The groups say Mr. Zuma may have overstepped his constitutional powers with the decision earlier this month. They said Wednesday that they will seek a hearing in Pretoria's High Court.

President Zuma announced June 3 that he was extending the term of Chief Justice Sandile Ngcobo by five years, until August 2016.

The judge had been due to step down two months from now, after 12 years as a Constitutional Court judge.

The groups filing the challenge are the Center for Applied Legal Studies and the Council for the Advancement of the Constitution.

They say Mr. Zuma's action infringes on the “separation of powers, rule of law and the independence of the judiciary” that form the basis of South Africa's constitutional structure.

They also say it could lead to the perception that the chief justice is beholden to the president.