WASHINGTON (CNN) -- State Department Counselor Philip Zelikow announced plans to resign, leaving Secretary of State Condoleezza Rice's without one of her closest advisers as the administration searches to a new approach to the turmoil in the Middle East, Rice announced Tuesday.

In his resignation letter, Zelikow cited "some truly riveting obligations to college bursars" for his children's tuition and said he remained available to help the administration as best he could.

"These are challenging times and there is much to be done," Zelikow wrote to Rice on Monday. "But I know you already see the challenges, and the opportunities too."

His resignation will be effective on Jan 2. He will return to the University of Virginia where he holds a endowed chair as a history professor which has been held vacant for four years.