In this work we report the performance of a fiber optic sensor towards ammonia and nitric oxide detection. The device is based on a Fabry-Perot interferometer which is fabricated by coating the tip of an optical fiber by means of sputtering with tin oxide (SnO2). The main advantage of this structure is its small size, which offers the possibility of making point to point measurements. Besides, this kind of optical devices enables working with the wavelength detection method in the UV-VIS-NIR range. The device was tested at room temperature under nitrogen flow with different concentrations of ammonia or nitric oxide in order to obtain the response of the sensor. The maximum sensitivity for ammonia concentration ranging from 0 ppm to 500 ppm is 4 pm/ppm, whereas wavelength shift while changing nitric oxide concentration from 0 to 375 ppb was 1.2 nm, which corresponds to a sensitivity of 10pm/ppb.