Image from @ThinkAgain_DOS, the U.S. Government Twitter account trolling ISIS U.S. Department of State

Severed heads. Mass executions. Images of 9/11. They are all standard fare on one of the most disturbing hashtags ever to hit Twitter, #calamitywillbefallUS.

Islamic State of Iraq and Syria (ISIS) militants are taking credit for the hashtag, intended to warn the United States and its allies of the consequences of using air raids against the Sunni militants attempting to establish a multistate caliphate across Syria and Iraq.

The hashtag has been mentioned nearly 24,000 times in the past four days, but Twitter isn’t taking down the images, and the U.S. Department of State may not be asking them to. Rather, the U.S. government is using the chain of posts as an opportunity to talk directly to ISIS militants and their sympathizers through its own account, @ThinkAgain_DOS.

The @ThinkAgain_DOS account is part of the Center for Strategic Counterterrorism Communications, a sector of the State Department established by executive order by President Obama in 2011. Its purpose is to “counter terrorist propaganda and misinformation about the United States across a wide variety of interactive digital environments that had previously been ceded to extremists.”

Ambassador Alberto Fernandez described the mission in a speech in December as "poking holes in the daily narrative" that extremist groups tell the world about themselves.

As the New York Times points out, ISIS militants are using the tactics of viral marketers, clumsily hacking trending topics and Twitter memes to advance their cause. Last month, for example, Islamic militants attached an image of a decapitated head with the Twitter message “This is our ball … It is made of skin #WorldCup.”

Nu Wexler, a spokesperson for Twitter, said the messaging service does not proactively monitor tweets, which roll in at a rate of 1 billion every two days. Rather, it responds to complaints of accounts that violate its terms of service, which include specific threats of violence.

Twitter can suspend accounts that are routinely in violation, but they do not block hashtags, meaning #calamitywillbefallUS will be around as long as it's attached to Twitter posts. While disturbing, the hashtag does serve a purpose in that it draws out militants and their simpathizers and allows the State Department and others to track and interact with them.

That's a different approach from a few years ago, when the policy was not to engage in any way that might disseminate the terrorist message. "The goal is to engage with and change the conversation, because it is happening," a State Department spokesman said.