The Democratic Republic of Congo opposition leader Moise Katumbi on Thursday warned President Joseph Kabila risked plunging the country into an "even worse crisis" by letting doubt linger over when he will step down.

Kabila, who has been in power since 2001, is widely believed to be considering casting aside the constitution to take a third term in office after his tenure expires at the end of the year.

In a speech to mark DR Congo's independence day this week, the president gave no hint that he would depart on December 19, declaring only that: "Nothing can stop the passage of future elections."

Political crisis

"This missed opportunity, and the uncertainty sustained by (Kabila's) speech risk instead plunging the country into even deeper crisis," said Katumbi, who came second to the president in 2011 polls that were marred by fraud.

DR Congo has been in political crisis since then, but tensions have bubbled over into deadly unrest in recent months as fears have grown the 44-year-old president would hold onto power.

Tensions rose further when the country's Constitutional Court ruled in May that Kabila could remain in a caretaker capacity beyond December until elections are held.

Kabila's supporters want elections, due later this year, to be delayed for two to four years. No date has yet been set for the polls.

Last month, opposition parties forged an alliance to demand Kabila quit his post in December, warning that him staying on would be tantamount to a "constitutional coup d'etat".

READ: DR Congo opposition protests delay in elections, extension of term limits

Katumbi was an ally of Kabila for a decade but quit the ruling party in September over the president's plans to split several provinces into four separate entities.

The football magnate then left the country on May 20, a day after the government announced he would be tried for endangering state security.

READ: DR Congo announces probe into opposition use of US mercenaries

In June, he was sentenced to three years in jail last month over a real estate dispute, effectively making him ineligible to stand in any elections held this year.

Kabila said in a statement the charges were trumped up, and created to "prevent me standing for president".

"Neither the sentences, nor the harassment to which I have been subject, or the physical violence, can shake our determination," he said. "On December 20, 2016, Joseph Kabila will no longer lead the DRC".