In recent experiments at JET, ICRF waves were used to either stabilise or destabilise sawteeth, and consequently control the formation of the seed island responsible for the appearance of Neoclassical Tearing Modes (NTMs) known to limit the tokamak performance. Two scenarios were studied in deuterium plasmas with a hydrogen concentration of 5 to 10%. Firstly, using a toroidal magnetic field B0 of 2.5T, the fundamental hydrogen cyclotron resonance layer (w=wcH) was positioned at the High Field Side (HFS) at the sawtooth inversion radius (Rinv~Rq=1). With +90o phased wave, monster sawteeth were obtained that triggered NTMs. Secondly, ICRF waves at the 2nd harmonic hydrogen cyclotron resonance (w=2wcH) positioned at the Low Field Side (LFS) with B0 between 1.2T and 1.6T, was used to reduce the sawtooth period and increase the NTM bn onset level.