Johannesburg - About 2 400 mineworkers at Anglo Platinum's [JSE:AMS] (Amplats) Thembela mine in Rustenburg are being prevented from exiting the shaft, the company said on Friday."We confirm that this is as a result of the suspension of four shop stewards for inappropriate behaviour which is against our behavioural procedure," said Amplats' spokesperson Mpumi Sithole.The Association of Mineworkers and Construction Union (Amcu), could not be reached for comment on the company's claims.Earlier, the French news agency Agence France-Presse (AFP) reported that the workers were staging a sit-in strike over the suspension of four union leaders."They don't want to come out from underground because they want their leadership's suspension lifted," Amcu official George Tyobeka was quoted as saying. He said 3 000 to 4 000 people were on strike.The four Amcu leaders suspended were reportedly accused of submitting fraudulent membership applications in an attempt to inflate union membership numbers.On Friday afternoon, Sithole said the company's protection services teams was on the ground "attending to the situation"."Management is currently engaging with recognised unions at regional and at head office levels to resolve the situation."Amcu and its rival union, the National Union of Mineworkers (NUM), have been struggling for dominance at the mines, which has resulted in violent strikes and assassinations.Last year, the police shot dead 34 miners at the neighbouring Lonmin platinum mine in Marikana.Eight NUM members were recently suspended at Lonmin for alleged union membership fraud, AFP reported.