Brain computer interfaces (BCI) that use the steady-state-visual-evoked-potential (SSVEP) as neural source, offer two main advantages over other types of BCIs: shorter calibration times and higher information transfer rates. SSVEPs elicited by high frequency (larger than30 Hz) repetitive visual stimulation are less prone to cause visualfatigue, safer, and more comfortable for the user. However in the high frequency range there is a practical limitation because only fewfrequencies can elicit sufficiently strong SSVEPs for BCI purposes.We bypass this limitation by using only one stimulation frequency and different phases. To detect the phase from the recorded SSVEP, weuse spatial filtering combined to phase synchrony analysis. We developed an online BCI implementation which was tested on six subjects and resulted on an average accuracy of 95.5% and an average bit rateof 34 bits-per-minute. Our approach has the advantage of entailing only minimal visual annoyance for the user.