Cytarabine and Clofarabine in Treating Patients With Refractory or Relapsed Acute Myeloid Leukemia or Acute Lymphoblastic Leukemia

RATIONALE: Drugs used in chemotherapy, such as cytarabine and clofarabine, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more cancer cells. PURPOSE: This phase I trial is studying the side effects and best dose of clofarabine when given together with cytarabine and to see how well they work in treating patients with refractory or relapsed acute myeloid leukemia or acute lymphoblastic leukemia.