The energetics and ultrafast dynamics in the H atom transfer configuration of ammonia dimer and trimer clusters have been studied. The clusters are first excited to the electronic (A) over tilde state with a 208 nm femtosecond laser pump pulse. This state is allowed to relax for about 1 ps during which the H-transfer state is formed which is then electronically excited by a time-delayed infrared control pulse at 832 nm and finally ionized with a third femtosecond probe pulse at 416 nm. We have also performed complementary theoretical studies elucidating the experimental findings. For the dimer in the excited NH4(3p)...NH2 ((X) over tilde) state the time-dependent ion signals reveal an isotope-independent short lifetime of about tau(6) = (130+/-60) fs which can be explained by a curve crossing with the repulsive NH4(3s)...NH2((A) over tilde) state, whereas the trimer signal persists on a time scale being more than one order of magnitude longer and exhibits a very large isotope effect. This is interpreted as being due to internal conversion from the excited state NH3NH4(3p)...NH2((X) over tilde) back to the NH3NH4(3s)...NH2((X) over tilde) ground state. The analysis of the corresponding photoelectron spectra also confirms the transition energies between the electronic states involved, e.g., DeltaE[NH4(3s-->3p)...NH2]=1.5eV and DeltaE[NH3NH4(3s-->3p)...NH2]=1.2 eV, as determined by our ab initio calculations.