Is myelodysplastic syndrome a form of leukaemia?

According to the current clinical WHO staging criteria for myelodysplastic syndromes, all staging of the disease is not leukaemic, but myelodysplastic syndromes are at high risk of transformation to leukaemia and were once referred to as pre-leukaemic.
The reduction in blood cells caused by myelodysplastic syndrome is extremely difficult to treat and is also a highly malignant disease.