The UN peacekeeping mission, Monuc, has confirmed the taking of Rumangabo and said that heavy artillery exchanges were continuing. No figures on casualties have been given.
The CNDP took the Rumangabo camp earlier this month but later abandoned it.
The group also took control of Virunga National Park - home to more than a quarter of the world's mountain gorillas - 5km away from the Rumangabo camp, officials said.
"Rebels loyal to dissident general Laurent Nkunda took over the headquarters of the Virunga National Park and the sector where the gorillas are to be found after heavy fighting with the DRC army in the early hours of the morning," a statement released by Emmanuel de Merode, the park director, said.
"More than 50 rangers were forced to flee through the forest.
"The taking of our headquarters at Rumangabo by the rebels is unprecedented in all these years of fighting."
Fighters battling in the eastern Nord-Kivu province regularly hide in the park, on the border with Rwanda and Uganda.
The CNDP have accused the DRC's army (FARDC) of attacking them during the past few days and Monuc of taking the side of the army and Rwandan Hutu fighters.
"If Monuc was not going to stop the FARDC offensive, they should not stop our counter-attack," Bertrand Bisimwa, a CNDP spokesman, said.
"We have no alternative but to continue fighting. Wherever we are shot at from ... we will go.
"MONUC should stop messing with us."
The government has accused Rwanda of backing the CNDP's forces.
The US Security Council last week rebuked Nkunda for encouraging a national rebellion and called on all armed groups in the East African county to put down their weapons.
The east of the country has seen increased violence since a peace deal collapsed in August. Since then about 200,000 people have been displaced, the UN said on Friday.
A civil war raged in the DR Congo from 1998 to 2003, with an estimated 5.4 million people dying due to the fighting and associated humanitarian disaster.