The Party for the Unity and Safeguarding of the Integrity of Congo (PUSIC) said on Monday the massacre was carried out by fighters from the majority Lendu tribe.
It also accused government troops of the Democratic Republic of Congo (DRC) of taking part in the attacks.
“They killed 352 civilians, men, women and children, 37 of whom were at the Tchomia Hospital”, said Kisembo Bitamara, spokesman of PUSIC that mainly represents the minority Hema tribe.
The new toll could not be immediately confirmed by independent sources.
The massacre occurred in the town of Tchomia, about 55 kilometres east of Bunia and began on Saturday at 0300 GMT and lasted the whole morning, according to Bitamara.
Bunia is the main town of Ituri that is rich with some 200 small-scale gold mines. Clashes between Hema and Lendu fighters have been erupting in the town in recent weeks, resulting in the death of hundreds of civilians.
Bitamara said that some 2,500 attackers rampaged Tchomia's main trading centre and killed 253 people, and went to PUSIC leader’s residence, killing 22 of his relatives. The leader, Kawa, was not there.
In the hospital, 37 people had their throats cut or slashed, he said.
PUSIC found bodies of six government soldiers and 12 Lendu fighters who were killed by the Hema tribes, according to the party's spokesman. Fourteen of the Hema fighters were killed too.
“Kinshasa is behind this attack… The Lendu did not have mortars and machine guns before they came from Kinshasa”, Bitamara said referring to the government which is based in the African country’s capital.
Government officials and Lendu commanders could not be reached for comment.
UN was warned
Amnesty International and Human Rights Watch jointly called on the United Nations on May 21 to authorise the deployment of a rapid reaction force to prevent mass killings of civilians in Ituri.
They said the UN Observation Mission in Congo (MONUC) has been unable to adequately protect civilians.
The massacres came shortly after Uganda withdrew its troops from the area on Friday.
“I am made to understand there were Lendu militias in the surrounding hills waiting for us to pull out”, Ugandan Brigadier Kale Kaihura, who was overseeing his troops withdrawal from the DRC.
As Uganda started pulling out its troops in April, there was a power vacuum that was quickly filled by local militias who fought each other to take control of Bunia.