"I am deeply shocked by the images of civilians killed in Bucha, Ukraine," Guterres said in a tweet on Sunday.
"It is essential that an independent investigation leads to effective accountability," he added.
Guterres posted his comments on Twitter a day after witnesses and officials said that Russian troops killed almost 300 civilians as they withdrew from the town near Kyiv, the Reuters news agency reported.
Ukrainian President Volodymyr Zelenskiy on Sunday accused Russia of committing genocide in his country.
Ukraine's Foreign Minister Dmytro Kuleba said in a tweet that the "Bucha massacre was deliberate" and that the "Russians aim to eliminate as many Ukrainians as they can."
While on a visit to Poland, Kuleba demanded "new devastating" sanctions against Russia from the G7 group of nations, listing an "oil, gas and coal embargo;" the closure of "all ports to Russian vessels and goods;" and a decision to "disconnect all Russian banks" from the SWIFT bank messaging system.
He also tweeted that the "Bucha massacre" proved that Russia's "hatred towards Ukrainians is beyond anything Europe has seen" since World War II.
Polish Justice Minister Zbigniew Ziobro said last month that prosecutors in Poland had collected more than 300 witness statements as part of an ongoing investigation into war crimes committed by the Russian invaders of Ukraine.
Polish Foreign Minister Zbigniew Rau told the United Nations in early March that Russia's authorities were showing contempt for human rights in their assault on Ukraine and its people.
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Source: IAR, PAP, Reuters