FRANKFURT, Nov 29 (Reuters) - German drugmaker Merck KGaA and U.S. partner Pfizer have been granted priority review status in the United States as they seek approval to fight a rare and aggressive form of skin cancer with immunotherapy drug avelumab.

The U.S. Food and Drug Administration has accepted the request for approval and will review the trial data for the drug against metastatic Merkel cell carcinoma within six months instead of the usual 10, Merck said on Tuesday.

Avelumab belongs to a new generation of biotech drugs that stop some tumours from hiding from the immune system, similar to Merck & Co's Keytruda or Roche's Tecentriq.

The future of Merck KGaA's pharmaceuticals division rests largely on avelumab, after the business suffered development setbacks in several other medicines and amid rising competition for its established brands, such as Rebif against multiple sclerosis.

The FDA has previously granted Merck and Pfizer other support schemes for the drug, such as breakthrough therapy designation.

(Reporting by Ludwig Burger; Editing by Georgina Prodhan)