Twitter, with its strict 140-character limit, was never going to be the best medium to make a nuanced point about Middle East politics. But Octavia Nasr gave it a go.

The cost was great: Nasr was fired as CNN's senior Middle East editor after 20 years with the US-based news channel.

The offending tweet was sent on Sunday morning following the death in Beirut of Grand Ayatollah Mohammed Hussein Fadlallah, who was instrumental in the establishment of Hezbollah in Lebanon. Using her official CNN Twitter account Nasr wrote: "Sad to hear of the passing of Sayyed Mohammad Hussein Fadlallah. One of Hezbollah's giants I respect a lot."

The tweet was immediately picked up by supporters of Israel, to which the Islamist group is bitterly opposed. The Simon Wiesenthal Center in the US released a statement demanding Nasr "apologise to all victims of Hezbollah terrorism whose loved ones don't share her sadness over the passing of one of Hezbollah's giants".

The text was swiftly removed from her Twitter feed, but by then it had been heavily circulated, with criticism mounting.

Nasr responded on Tuesday with a blog on the CNN website, calling her initial message "simplistic" and "an error of judgment". Her respect for the ayatollah, who she had interviewed for Lebanese television in 1990, was owing to his stance on women's rights, notably his demands that "honour killings" stop, she explained.

But this was not enough. The next day, Nasr was reportedly called in to see her bosses at CNN's headquarters in Atlanta. The New York Times quoted an internal memo from a senior vice-president, Parisa Khosravi, which said: "We have decided that [Nasr] will be leaving the company."

The memo added: "At this point, we believe that her credibility in her position as senior editor for Middle Eastern affairs has been compromised going forward."

The company has not confirmed the news, saying only that the tweet "did not meet CNN's editorial standards". A spokesman added: "This is a serious matter and will be dealt with accordingly." Nasr's Twitter account has fallen silent.

Fadlallah, 74, was Hezbollah's spiritual leader when it formed after Israel invaded Lebanon in 1982, though he later distanced himself from the group's ties with Iran.

Nasr, who appeared on camera and worked behind the scenes at the TV station, soon realised her mistake, writing on her blog: "Reaction to my tweet was immediate, overwhelming and provides a good lesson on why 140 characters should not be used to comment on controversial or sensitive issues, especially those dealing with the Middle East."

While her tweet attracted controversy, a tribute to Fadlallah came from another seemingly unlikely source: the UK ambassador to Beriut.

Frances Guy, who has headed the mission since 2006, wrote on her official Foreign Office blog: "Lebanon is a lesser place … the world needs more men like him, willing to reach out across faiths, acknowledging the reality of the modern world and daring to confront old constraints. May he rest in peace."

Comments beneath the post were mainly positive, although one read: "Her esteemed predecessors, such as Sir John Gray, lived in mortal fear of being blown up by Fadlallah's Hezbollah hoods. So much for the 'admired Shia leader' she refers to above."

Nasr is one of the more high-profile victims of a phenomenon known as "twittercide". A notable UK casualty was Stuart MacLennan, a Scottish Labour candidate deselected a month before the election for using Twitter to call old people "coffin dodgers" and David Cameron "a twat".

Last month an Irish exam supervisor was dismissed after using his phone to tweet: "I do pity the girls that have me supervising, im young, handsome & probably very distracting ha ha". Meanwhile a columnist for Australia's Age newspaper lost her job after tweeting her wish that an 11-year-old child TV star "gets laid".