Police have spoken only of "hundreds" killed in the attack on Sunday, but access to the remote farming town has been very limited because of the threat of reprisals.
"We buried a total of 630 people yesterday, 1500 people were injured and 600 were taken for emergency treatment," said community leader Abd Allahi Abd Allahi.
Abd Allahi showed a Reuters correspondent a foul-smelling area of freshly turned earth in the town where he said some of the people had been buried. Nearby a mosque had been gutted by fire and one of its walls had collapsed.
Nigeria is the world's seventh largest oil exporter.
Ethnic fighting has hit oil production in the past, but Yelwa is hundreds of kilometres from any oilfields. Nigerian Red Cross worker Umar Mairiga said he saw another mass grave where 350 people had been buried.
Sunday's murders brought the toll from three months of fighting between the Christian Tarok and Muslim Fulani to 1000 in the conflict rooted in competing claims over fertile land at the heart of Africa's most populous nation.
It is also fuelled by religious and ethnic differences.