Micro-blogging site Twitter said its domain name system records were temporarily "compromised" Thursday night, but that the problem was fixed later.
Twitter.com was redirected for a while, but some applications were working, the company said on its blog.
Twitter was investigating the issue and would provide more information when available, it said.
According to media reports, the site was hacked for about two hours Thursday night by a group calling itself the "Iranian Cyber Army."
The group also hacked an Iranian opposition Web site, the reports said.