Officials from the party of Democratic Republic of Congo's former President Joseph Kabila have been invited to appear before a military prosecutor on Monday in a sign of political tensions over Rwanda-backed rebels' advances in the east.

The exact reason for the invitations was not clear, said Jean Mbuyu, a lawyer for the officials and former security advisor to Kabila.

But President Felix Tshisekedi, who once formed an awkward power-sharing deal with Kabila, has recently accused him of sponsoring the M23 rebels who have seized east Congo's two biggest cities since January.

Kabila has also reached out to opposition politicians and civil society members to discuss the country's political future, amid criticism of Tshisekedi's response to M23's military campaign.

The military prosecutor's office sent about 10 invitation letters to members of Kabila's People's Party for Reconstruction and Democracy, though only three people were expected to appear for questioning in the capital Kinshasa on Monday, Mbuyu said.

They included Aubin Minaku, vice president of the party and former president of the National Assembly, and Emmanuel Ramazani Shadary, a former interior minister and presidential candidate, Mbuyu said.

"We're going to listen because the invitation contains no reason," he said.

Minaku said the officials would respond to the invitations "to avoid any suspicion" and denied links to M23 or any other armed groups.

"We are taking a purely Republican approach, not one of rebellion," he told Reuters, using "Republican" to indicate loyalty to the country.

"We clearly denounced any illicit presence of foreign forces," he said.

The latest M23 advance is the gravest escalation of a long-running conflict rooted in the spillover into Congo of Rwanda's 1994 genocide and the struggle for control of Congo's vast mineral resources.

Rwanda denies providing arms and troops to M23, and says its forces are acting in self-defence against the Congolese army and militias hostile to Kigali.