The fighting started days ago and is continuing in the Porgera Valley, near the site of a landslide in May estimated to have killed more than 2000 people, according to the South Pacific island nation's government.
The UN's humanitarian adviser for PNG, Mate Bagossy, said the death toll of at least 20 as of Sunday was based on information from community members and local authorities in Enga province in the nation's mountainous inland.
"We have confirmed that at least 20 people have been killed, but as per the last news that I have, it's likely up to 50 people," Bagossy told The Associated Press.
"The fighting is continuing."
"Today some security forces have started moving in ... so it remains to be seen what effect this will have," Bagossy said, referring to the army and police.
Bagossy did not have information on the numbers wounded.
Pope Francis visited PNG on September 6 during his 11-day Asia-Pacific trip and urged residents to work at being good to one another.
He said coming together harmoniously to overcome rivalries would help to "drive out fear, superstition and magic from people's hearts, to put an end to destructive behaviours such as violence, infidelity, exploitation, alcohol and drug abuse, evils which imprison and take away the happiness of so many of our brothers and sisters."
It was a reference to the tribal violence over land and other disputes that have long characterised the country's culture but have grown more lethal in recent years.
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