"The mission mounted two patrols into Wau today and said it had observed the bodies of 16 civilians in a hospital. There were ten people who had been injured," the statement said.
UNMISS said SPLA troops, tanks and equipment moved towards the south-western part of Wau late last week.
After an ambush south of the city left a number of government SPLA soldiers dead on Sunday, fighting spread to Wau.
Wau is located in a region that has repeatedly changed hands between government troops and rebels loyal to former vice president Riek Machar since the country descended into civil war in 2013.
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The town itself is held by forces loyal to President Salva Kiir.
Civil war broke out in South Sudan in 2013 after a power struggle between foes Machar and Kiir.
At least 1.7 million people have fled the country because of the war, and 1.9 million are internally displaced.
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