YouTube and a popular Russian social network, Odnoklassniki, were inaccessible through many Internet service providers in Tajikistan on December 30.
Internet users across the country said they had been unable to open the websites since the early morning. Some people say they could access them through proxies.
An official from a major ISP, who did not want to be named, said the state Communication Services agency had verbally instructed ISPs to "temporarily" block the two websites for "preventative reasons."
The agency, however, denied the claim.
YouTube and Odnoklassniki, as well as Facebook and Gmail, have been blocked in Tajikistan several times this year.
SMS services and hundreds of websites were largely inaccessible for several days after the opposition movement Group 24 used social media to call for an antigovernment protest in the capital, Dushanbe, on October 10.