Story highlights Twitter says the move will be on a case-by-case basis

It will still make the tweets visible to the rest of the world

Users will be notified if their tweets are deleted

One observer calls it censorship

Online social networking site Twitter said Thursday it will begin deleting users' tweets in countries that require it -- but it will still keep those deleted tweets visible to the rest of the world.

The move is significant because, until now, the only way Twitter could comply with countries' limits was to remove the content globally.

Twitter said it will now delete tweets only "reactively" and on a case-by-case basis and will let the affected user know why the content is being withheld.

"We hold freedom of expression in high esteem and work hard not to remove Tweets," Twitter spokeswoman Jodi Olson wrote in an e-mail to CNN.

"And just to be clear, this is not a change in philosophy and there are still countries to which we will not go," she said.

As examples of countries where tweets may have to be restricted, Twitter mentioned Germany and France, which ban pro-Nazi content. Twitter will operate in those countries while censoring pro-Nazi tweets, for example.

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But some countries are so restrictive with freedom of expression that Twitter can't exist there at all, the company said.

"One of our core values as a company is to defend and respect each user's voice," Twitter wrote in a blog entry Thursday. "We try to keep content up wherever and whenever we can, and we will be transparent with users when we can't. The Tweets must continue to flow."

But one observer said the move is blatant censorship.

"There's no way around that. But alas, Twitter is not above the law," according to Jillian York, the director for international freedom of expression at the Electronic Frontier Foundation, which defends free speech and privacy online.

York said she acknowledges that Twitter, like other companies hosting user-generated content, has at some point faced a government order or request to take down content.

"Twitter has two options in the event of a request: fail to comply, and risk being blocked by the government in question, or comply (read: censor). And if they have 'boots on the ground,' so to speak, in the country in question? No choice," she wrote.

The company is simply "doing its best in a tough situation," she said.