MONROVIA (Reuters) - Sable Mining Chief Executive Officer Andrew Groves said on Friday that his indictment this week by a grand jury in Liberia in relation to a corruption investigation was politically motivated.

President Ellen Johnson Sirleaf last month ordered an inquiry into London AIM-listed Sable's unsuccessful attempt to acquire an iron ore concession in northern Liberia after the watchdog group Global Witness made accusations of wrong-doing.

"Andrew Groves was surprised to learn from media reports that a grand jury in Liberia appears to have indicted him on unspecified charges," Groves's public relations firm said, adding he had not been given a chance to respond to the charges.

Four people, including Parliament speaker Alex Tyler, chairman of Sirleaf's Unity Party Varney Sherman and deputy minister of Lands, Mines and Energy Ernest C.B. Jones, were indicted last month in that case along with Sable Mining.

They are accused of having used their positions to amend Liberia's public procurement and concessions law. They deny the charges and have been released on bail.

"Mr Groves believes that these allegations are politically motivated ahead of the imminent presidential elections, and are specifically designed to undermine the candidacy of Varney Sherman ... and Alex Tyler," Grove's statement read.

Liberia will hold a presidential election next year to replace President Sirleaf, who cannot run again due to a constitutional term limit.

Sherman has acknowledged that his law firm was employed by Sable Mining.

The indictment alleges Tyler requested and received $75,000 to help amend the concessions law.

(Reporting by James Harding Giahyue; Writing by Joe Bavier; Editing by David Gregorio)