Fatou Bensouda's announcement comes after Kiev earlier this month accepted the court's jurisdiction to probe war crimes, including those committed in bitter fighting between Ukrainian forces and pro-Moscow rebels that has left thousands dead so far.
The preliminary probe by the ICC, based in The Hague, aims to determine whether a full-out investigation is warranted.
An extended probe could also cover the MH17 plane crash in July 2014 in which 298 people - the majority of them Dutch - died when a Malaysia Airlines passenger jet was downed over eastern Ukraine, observers say.
Ukraine in April 2014 gave the ICC the green light to probe alleged crimes committed between November 21, 2013, when pro-EU demonstrations erupted in Kiev, and February 22, 2014, when pro-Moscow president Viktor Yanukovych was ousted.
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Earlier this month it gave the ICC the go-ahead to extend its investigation beyond February 2014 to include the deadly civil conflict that has wracked the east of the country since April 2014.
Kiev said its parliament decided to extend the court's jurisdiction to probe "crimes against humanity and war crimes committed by senior officials of the Russian Federation and terrorist" (separatist) leaders.
Civil rights groups however say that since February 2014 numerous war crimes and crimes against humanity are alleged to have been committed in eastern Ukraine by both sides in the conflict.
The ICC's Bensouda stressed today that her open-ended probe was "independent and impartial".
The United Nations estimates that the 17-month-old conflict in eastern Ukraine has claimed the lives of nearly 8,000 people and injured almost 18,000, most of them civilians.
The UN human rights office, in a report earlier this month, not only underscored the brutality of the fighting in the rebel-controlled eastern territories of Donetsk and Luhansk, but also drew attention to "a persistent pattern of arbitrary and incommunicado detention" by Ukrainian forces.