Specifically, Twitter has had to take defensive actions that are preventing some third-party Twitter applications from communicating with the company's API (application programming interface).
In addition, many users can't post Twitter messages via SMS (Short Message Service), as Twitter continues to defend itself against the attack, which the company described as "ongoing" in the blog.
"Due to defensive measures we’ve taken against the ongoing denial-of-service attack, some Twitter clients are unable to communicate with our API, and many users are unable to tweet via SMS," reads the . "We are working as quickly as possible to restore our full service."