A US-based opposition group labelled "terrorist" by Vietnam on Monday blamed the communist government for a cyber attack that it said had crippled its website.
"Beginning on August 13, hackers launched a sustained attack against www.viettan.org," said Viet Tan, also known as the Vietnam Reform Party, which campaigns for peaceful political change in the authoritarian state.
On August 13 the "Hanoi government's firewall" was lifted to allow a "botnet", or network of compromised computers, to take down the Viet Tan website, the group alleged in a statement.
"This is further evidence that the communist authorities of Vietnam are behind the ongoing hacker attacks against pro-democracy websites and blogs," it said.
Authorities could not immediately respond to a request for comment.
Viet Tan said the attack, known as a distributed denial-of-service (DDoS) operation, was carried out by a botnet using 77,000 Internet protocol addresses, most of which originated in Vietnam.
Since early last year rights activists and other observers have accused Vietnam of using cyber attacks in a more aggressive stance towards politically sensitive Internet sites.
Vietnamese users of Facebook also report difficulties in accessing the social networking site.
The alleged attack on Viet Tan's website occurred shortly after a member of the group was jailed by a court in Ho Chi Minh City. French-Vietnamese lecturer and blogger Pham Minh Hoang, 56, was convicted of attempted subversion, a decision France said it deeply regretted.
Internet use has grown rapidly in Vietnam, with more than 30 percent of the population online, the government says.