Rwanda has accused UN-backed DRC forces of shelling its territory, but said it won't respond militarily to what it called Kinshasa's "provocation".
AngloGold Ashanti says its Mongbwalu gold mine in the DRC faces the obstacle of ex-fighters already mining the area who do not want to leave.
Human Rights Watch says DRC security forces have killed 24 people since President Joseph Kabila won a disputed presidential election.
Joseph Kabila has conceded "mistakes" have been made in recent elections but rejects criticism by monitors that the results lacked credibility.
African Union observers have urged candidates in DRC's elections to accept the outcome of this week's polls, insisting they were well managed.
At least 20 UN workers were killed when their plane crashed on Monday while trying to land in the Democratic Republic of the Congo's capital Kinshasa.