Navi Pillay, the U.N. high commissioner for human rights, criticized the international community for failing to honor 2005 commitments to step in where national governments like Syria's were failing to protect citizens or otherwise suspected of atrocities.
Moscow during the weekend opposed a U.N. Security Council resolution on Syria, saying Russians officials feared such a move would lead to an international military operation like last year's war in Libya.
Putin said the Kremlin condemns the violence in Syria "wherever it comes from." Outside countries, however, should "not interfere under any circumstances," he was quoted by Russia's state-run news agency RIA Novosti as saying.
"The (Syrian) people must decide their future themselves," he said.
Human Rights Watch said it spoke with at least eight witnesses to the government siege on Homs, describing the assault as "brutal" and "horrific."
U.N. Secretary-General Ban Ki-moon said late Tuesday he "deeply" regretted the stalemate at the Security Council.
"I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighborhoods, is a grim harbinger of worse to come," he said in a statement.