ICC prosecutor Fatou Bensouda said there had been "endless" instances of rape, murder, forced displacement, persecution and pillaging since 2012.
She said both sides had committed atrocities in what has become a religious conflict between mainly Muslim rebels and Christian militias.
Almost a quarter of the 4.6 million population have fled their homes.
The turmoil began in the run-up to the mainly Muslim Seleka rebel group seizing power in the majority Christian country in March last year.
Rebel leader Michel Djotodia resigned as president in January under intense diplomatic pressure but the killings continued.
Muslims had to flee revenge attacks from largely Christian militias, known as anti-balakas.
Last month, a power-sharing government was formed - but it has not eased tensions and the country has largely been split into a Muslim north and Christian south.
Ms Bensouda, who opened preliminary investigations in February, said her office had "scrupulously analysed relevant information from multiple reliable sources".
Other war crimes and crimes against humanity included attacks against humanitarian missions and use of child soldiers, the prosecutor said.
"The list of atrocities is endless. I cannot ignore these alleged crimes," she said in a statement.
"Let this be a message to would-be perpetrators in CAR and beyond: Such crimes will not be tolerated and will be met with the full force of the law."
Earlier this month, the UN took over peacekeeping operations from the African Union force.
The International  Criminal Court has opened a formal probe into war crimes in the Central African Republic (CAR).