Foursquare, Quora, Reddit and other popular websites were sluggish or knocked offline on Thursday because of problems with their Web host Amazon.

"Our usually amazing datacenter hosts, Amazon EC2, are having a few hiccups this morning, which affected us and a bunch of other services that use them," check-in site Foursquare said in a status message. "Thanks for your patience."

Popular content sharing-site Reddit said it was in "read-only mode" because of problems at Amazon.

Quora, a user-generated question-and-answer site, said it was working to get back online following an "unexpected outage" and a similar site, Formspring, said it was also affected by the Amazon outage.

Amazon is best known as an online retailer but the Seattle, Washington-based company is also a major provider of cloud-computing services, renting out space on its powerful servers to customers around the world.

Global technology companies are investing billions of dollars in cloud computing, which involves hosting information on the Web and providing it to customers on demand.

Others websites experiencing sluggish service or downtime because of the problems at Amazon were online brand management service Hootsuite and the website of Foreign Policy magazine.

Another Amazon-hosted website unavailable on Thursday was that of US-based online activist network Change.org, which said Wednesday that hackers based in China have disrupted an online petition signed by nearly 100,000 people which urges Beijing to free outspoken artist Ai Weiwei.

Change.org said its website has gone down intermittently since Monday due to distributed denial of service (DDoS) attacks originating in China.

In a typical DDoS attack, a large number of computers are commanded to simultaneously visit a website, overwhelming its servers, slowing service or knocking it offline completely.

Change.org said Thursday that there was nothing to suggest that the problems facing Amazon and the cyber attacks on its website were linked.

The status dashboard for Amazon Web Services on Thursday showed the company was having issues with its operations based in northern Virginia and blamed it on an unspecified "networking event."

"We are working as quickly as possible to add capacity," Amazon said in an update around noon (1600 GMT), about seven hours after the problems began.

About two hours later, Amazon said "a number of people have asked us for an ETA on when we'll be fully recovered."

"Our high-level ballpark right now is that the ETA is a few hours," Amazon said.

With its website down, Foreign Policy took to Twitter and added a dose of humor.

In a series of tweets with the hashtag "articles we would be printing if our website worked," the magazine suggested: "Think Again: Could Computing."

Amazon's hosting services were last in the news late last year after the company booted WikiLeaks off its servers, saying the website which leaked tens of thousands of US military documents and diplomatic cables had violated its terms of service.