"Freedom of the media is a vital element in democratic societies and journalists must be able to carry out their legitimate work without fear of harrassment or intimidation.
"Any action intended to intimidate independent journalism and or limit freedom of expression is in contradiction to UN human rights standards."
The US embassy said it was closely following case.
"We have raised on several occasions our deep concern over efforts to suppress independent news media, including the blocking of news websites, intimidation, and disappearances of journalists," the US embassy said in a statement.
"The United States believes that an uncensored and independent media available to all citizens is an indispensable part of any vibrant democracy and we call for an end to the harassment of these and all other journalists."The two websites are linked to Sri Lanka's main opposition United National Party.