Boris Johnson has paid tribute to the victims of Srebrenica to mark the 25th anniversary of the massacre in Bosnia and Herzegovina.
Around 8,000 Muslim men and boys were killed in 1995 by Bosnian Serb forces - the worst atrocity on European soil since the end of World War Two.
The prime minister said "we owe it to the victims" to remember Srebrenica and "to ensure it never happens again".
It comes after he was criticised for a 1997 article he wrote about Srebrenica.
On Friday, Mr Johnson was urged to apologise for his comments in which he described "these Muslims" as not "exactly angels".
In a letter from more than 100 Muslim representatives and 30 MPs, Labour's Tony Lloyd said there can be "no excuse for in any way blaming the victims of a genocide for its perpetration".
But Downing Street said the comments had been taken out of context.
Mr Johnson said in a video posted on Twitter on Saturday: "I want to join with you once more in mourning the victims of those terrible events, and to stand with the families in their fight for justice.
"As in so many cases from this conflict which brought violence and destruction across the western Balkans, many families still do not know what happened to their loved ones. Many perpetrators have still not been held to account.
"And there are those who would prefer to forget or deny the enormity of what took place. We must not allow that to happen."
Foreign Secretary Dominic Raab, who previously worked as a Foreign Office lawyer involved in bringing war criminals to justice at the Hague, also released a statement.
He said: "During my time in the Hague between 2003 and 2006, pursuing those responsible for this dark chapter in European history, I was reminded daily of the heinous cruelty perpetrated against the innocent.
"The UK is determined to end impunity and help rebuild those countries affected."
The massacre took place during the Bosnian War (1992-1995) when the Serb army was engaged in an ethnic-cleansing operation.
Thousands of Muslims sought safety in Srebrenica, which the UN was protecting with Dutch forces, but the area fell in July 1995 during a Serb offensive led by General Ratko Mladic.
Prince Charles had planned to personally pay his respects at nearby Potocari cemetery but the trip was cancelled due to the coronavirus pandemic.
In a recorded message, he said the "terrible events" of July 1995 were "a dreadful stain on our collective conscience".
He said: "The international community failed those who were killed, those who somehow survived and those who endure the terrible loss of their loved ones.
"By remembering the pain of the past and learning its lessons, we can together resolve that it must never happen again."