The president has said he wouldn't apologize for the dropping of an atomic bomb on the city. But it is a place to "remind ourselves that the job's not done in reducing the prospect of nuclear war in the future," he said.
US President Barack Obama visited Hiroshima's Peace Memorial Park on Friday, making him the first sitting US head of state to visit the Japanese city.
"71 years ago, death fell from the sky and the world was changed," the president said after laying a wreath.
"We come to ponder the terrible force unleashed in a not-so-distant past," Obama said. "We come to mourn the dead."
The US leader also called for a reduction in nuclear arms, saying: "We must change our mindset about war itself."
"We must reimagine our connection to one another as members of one human race," emphasized Obama.
Hiroshima was devastated after a US B-29 bomber dropped an atomic bomb on August 6, 1945.
The bombing killed tens of thousands of residents in seconds, totaling some 140,000 by the end of the year.
Three days later, US armed forces dropped another atomic bomb on the port city of Nagasaki, claiming the lives of over 70,000 people that year.
Prior to his Hiroshima visit on Friday, Obama met with Japanese and US troops at Iwakuni air station and said the visit was a reaffirmation of "the great alliance" between Japan and the United States.
"This is an opportunity to honor the memory of all who were lost in WWII," he told the troops. "It's a testament to how even the most painful divides can be bridged. How two nations can become not just partners but the best of friends."
Obama made it clear ahead of the visit that although he would honor the victims of WWII, he would not apologize for the decision taken by the late US President Harry S. Truman.
"I will not revisit the decision to use atomic weapons in Hiroshima and Nagasaki, but I will point out that Prime Minister (Shinzo) Abe and I coming to Hiroshima together shows the world the possibility of reconciliation," Obama said in remarks published by the Asahi newspaper on Friday.
Many people in US view the bombings as a necessary to end the war and save lives, although some historians question this belief.
Most Japanese believe the use of the A-bombs was unjustified.
rs, ls/kms (AP, AFP, Reuters, dpa)