The president of Democratic Republic of Congo's electoral commission said on Sunday that a vote to replace President Joseph Kabila will probably not be possible this year, violating a deal that let Kabila stay on past the end of his mandate.

Kabila's refusal to step down at the end of his second elected term in December sparked protests that killed dozens of people. The opposition quickly denounced commission president Corneille Nangaa's announcement on Sunday as a declaration of "war".

"The parameters at our disposal give us, more or less, reason to think that, in December, it will probably not be possible to stick to that date," Nangaa said in an interview on France's TV5Monde, referring to the year-end deadline.

Under the accord struck on Dec. 31 between Kabila's representatives and opposition leaders, Kabila, in power since 2001, is also barred from trying to change the constitution to stand for a third term.

However, Kabila has said only the electoral commission can schedule the vote once it finishes enrolling millions of voters. In an interview last month with German newspaper Der Spiegel, he also said that he had "promised nothing" in the December deal.

In response to Nangaa's comments, opposition leader Felix Tshisekedi wrote on Twitter that the commission president "had declared war on the Congolese people" and promised a full response by the country's main opposition bloc on Monday.