Reliable values of thermal conductivity of thin films made of GaN and its alloys are important for further development of nitride technology due to the problem of self-heating in GaN-based power transistors and optical devices. Using the differential 3ω technique we measured the thermal conductivity of AlxGa1-xN thin films (x=0 and 0.4) grown by the hydride vapor phase epitaxy. Thermal conductivity of the examined Al0.4Ga0.6N alloy, which is about 25 W/mK at 300 K, displays a rather unusual temperature dependence. A noticeable growth of the thermal conductivity with temperature up to 350 K is more characteristic for amorphous or completely disordered materials. The measured high-temperature thermal conductivity data are in good agreement with predictions based on the virtual crystal model. Obtained results are important for modeling the self-heating effects in GaN transistors and can be used for the device structure optimization.