* DRDGOLD says workers are on an illegal strike
* Union to keep pressing for 13 percent wage increase
(Adds details, union)
JOHANNESBURG, Sept 17 (Reuters) - South African medium-tier gold producer DRDGOLD Ltd said on Thursday it was seeking a court order to declare illegal a strike that has affected two of its mines.
"We have applied for an interdict so that the strike can be declared illegal. The National Union of Mineworkers (NUM) did not follow the right process (in calling the strike)," DRDGOLD's spokesman James Duncan told Reuters.
Members of the NUM downed tools at the Blyvoor mine on Tuesday night, while others stopped work at the Crown mine on Wednesday. Output from the mines has been affected but processing of gold from stockpiled ore continued, Duncan said.
NUM spokesman Lesiba Seshoka said the union had followed all the processes before declaring a strike and that it would not reduce its demand for a 13 percent pay increase, nearly twice South Africa's inflation rate of 6.7 percent.
DRDGOLD has so far offered an increase of between 4 and 7 percent.
"The ball is in their (management's) court to come back to the negotiating table, why must we come down?" Seshoka said when asked if the union would agree to lower its pay demand.
DRDGOLD's production for the year to the end of June from its continuing operations fell 20 percent to 247,690 ounces, mainly due to the suspension of underground mining at the East Rand Proprietary Mines (ERPM) and the decommissioning of the tailings deposition facility at Crown mine.
Harmony Gold Mining Co., which is ranked just above DRDGOLD in terms of output, produced 1.5 million ounces for the year to end-June.
Harmony, Gold Fields and AngloGold Ashanti successfully negotiated pay increases with workers without strikes. The three gold producers negotiate as one under the Chamber of Mines, and reached a two-year wage settlement paying workers in different categories increases of up to 10.5 percent.
DRDGOLD negotiates wages individually with its workers. (Reporting by Shapi Shacinda, editing by Anthony Barker)