On social media, people reported renewed difficulty accessing Spotify in Europe, as well as problems with photos and video on Twitter.
Dyn said it was now fighting a third attack, which is "well planned and executed, coming from tens of millions IP addresses at same time". White House Press Secretary Josh Earnest said the Department of Homeland Security was aware of the attacks, but that "at this point I don't have any information about who may be responsible for this malicious activity".
Another user wondered how DDoS was trending if Twitter was down, writing: "Wait a second.if Twitter is experiencing a DDoS attack, then how is DDoS trending??"
The company initially said the outage was limited to the Eastern United States.
This latest wave that took Twitter down, along with all the others, seems to be affecting other parts of the world, especially the West Coast of the US and also Europe.
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PayPal Holdings Inc said that the outage prevented some customers in "certain regions" from making payments. The attack is primarily aimed at Dyn, an internet infrastructure company headquartered in New Hampshire.
Affected sites included Twitter, Etsy, Github, Vox, Spotify, Airbnb, Netflix and Reddit.
A major internet outage left some of the world's most-visited websites including Twitter and Netflix out of action for hours. Dyn happens to serve most of the big names on the Internet, so an attack on their services affects a good chunk of the online world.
A DDoS attack - or denial of service - is an attempt to flood a website with so much traffic that it impairs normal service. Hackers could cause major disturbances since DNS providers are in charge of forwarding extensive volumes of internet traffic.