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Twitter, Spotify and Netflix are among some of the world's most popular websites targeted by 'hackers' in a major cyber attack today.

Users also reported being unable to access sites such as PayPal, Pinterest and AirBnB, while PlayStation users claim the network was down too.

US officials are investigating an alleged Distributed Denial of Service (DDoS) attack - a popular trick that floods a website's servers with requests and overloads it, according to reports.

The Department of Homeland Security told the Guardian it was probing what caused several leading websites, including the New York Times and the Wall Street Journal.

Reports suggest it is mostly those based on the US east coast who are unable to gain access, but some European users have also been affected.

(Image: Reuters)

The attack appears to have been focused on Dyn - a server provider for those websites.

The company said it was working to "mitigate" the issue and restore the service.

Dyn confirmed in a post on its website: “Starting at 11:10 UTC on October 21th [sic]-Friday 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure.

"Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available."

Some of the affected companies have separately confirmed the issue.

No one has yet confirmed responsibility for the attack.

It was suggested that there was a possible link to supporters of controversial Wikileaks founder Julian Assange.

However Wikileaks denied these claims, posting on Twitter this evening: "Mr. Assange is still alive and WikiLeaks is still publishing. We ask supporters to stop taking down the US internet. You proved your point."

What is a DDoS attack and how does it work?

A DDoS attack is an attempt to make an online service unavailable by overwhelming it with internet traffic from multiple sources.

This is achieved by creating a network of tens of thousands of compromised computers, known as a "botnet", to flood a website's servers with page view requests, leaving legitimate traffic unable to get through.

This huge volume of connection requests can quickly overwhelm a server and, in some cases, cause an entire website to crash.

How is a botnet created?

In order to add computers to a botnet, hackers must first take control of each machine individually.

They achieve this by exploiting vulnerabilities within the computer's operating system to install malicious software that provides them with remote access to the PC.

Once a machine has been compromised, it can be called into action by the hacker at a moment's notice.

How is a DDoS attack executed?

All the hacker needs to do is run a small program that communicates with all the computers in their control. They can then command those computers to start dialling out across the internet to a specific server or website.

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Although the impact of a DDoS attack can be cataclysmic for websites, they are relatively easy to execute for someone with the right technical expertise.

Building a botnet is the difficult and time-consuming part - so much so that some enterprising cyber criminals actually "lease" botnets to one another for spur-of-the-moment attacks.

Why do people carry out DDoS attacks?

People carry out denial of service attacks for a variety of reasons, and target a wide range of important resources, from banks to news websites.

Some "bedroom hackers" do it simply for the bragging rights, while cyber criminals often do it for blackmail.

(Image: Getty)

Gambling websites have traditionally been popular targets – with hackers threatening to overwhelm the site just before a big race, match or fight, when it would expect to be handling lots of bets from customers, unless the site pays a "ransom" to stay online.

It can also be a used as a weapon for those engaged in cyber warfare . In 2007, for example, Russian hackers were alleged to have carried out DDoS attacks on Estonian websites, in revenge for the country's decision to move a Soviet-era war memorial.