DARMSTADT, March 3 (Reuters) - Germany's Merck KGaA said underlying core earnings would stagnate or grow only slightly this year as it spends money on developing a new immunotherapy against cancer and as its injectable multiple sclerosis drug Rebif faces strong competition from oral treatments.

The outlook does not yet take into account the effect of the planned takeover of U.S.-based lab supplies maker Sigma-Aldrich , the company said on Tuesday.

Merck, a maker of drugs, high-tech chemicals and lab supplies, also said fourth-quarter earnings before interest, taxes, depreciation, amortisation (EBITDA) and one-offs rose 10.5 percent to 878 million euros ($983 million), surpassing the 870 million euros expected on average by analysts polled by Reuters.

($1 = 0.8932 euros) (Reporting by Ludwig Burger; Editing by Kirsti Knolle)