Blake, the top US diplomat for South and Central Asia, would raise the allegations with all sides after the decades-long civil war that ended in 2009 with the government's defeat of the separatist Tamil Tigers, sources told AFP.
"He will meet government officials, civil society representatives, university students and political leaders while in Sri Lanka," the US State Department said in a statement.
Blake was scheduled to start his visit with meetings on Monday with the Tamil National Alliance (TNA), the main political group representing the island's Tamil minority.
The Human Rights Council meeting in Geneva September 12-30 is due to be opened by United Nations High Commissioner for Human Rights Navi Pillay, and allegations of war crimes in Sri Lanka may be discussed during the session.
An Amnesty report last week cited eyewitness testimony and information from aid workers suggesting that at least 10,000 civilians were killed in the final military offensive that crushed the rebe