Supporters of the Islamic State group on Twitter have started a campaign of threats against Americans under the hashtag “#WeWillBurnUSAgain,” referring to the 9/11 attacks and past “lone wolf” attacks in Western countries, while also saying that they will repeat such attacks in future, according to SITE Intelligence Group, which tracks online activity of extremist organizations.

The content posted on Twitter under "#WeWillBurnUSAgain" includes text messages, images, videos and past ISIS press releases. The hashtag was first used by the Twitter account of “Rabitat al-Ansar,” a pro-ISIS media group, on Wednesday, marking the beginning of the anti-American campaign, SITE Intelligence Group reported, adding that the hashtag was announced as part of a message in a long series of anti-American tweets.

“We swear that lone wolves are present in all countries of the world and lurking for you. What happened in France is not far from you and will be repeated, but this time in the streets of American cities,” the message read.

In subsequent tweets, the account specified the date and time for the campaign.

“And let your slogan to be today as ‘I shall not survive if the worshipper of the Cross survived’ and Allah permitting, the Media Campaign will be on Friday from 4:30 through,” the message added.

According to SITE, the hashtag has already attracted more than 15,000 users, with majority of threats posted for lone wolf attacks -- violent acts committed by someone alone in support of some group, movement or ideology.

“In a time of lone wolf attacks, and Americans and other Westerners pledging to the Islamic State, this kind of campaign should not be taken lightly,” SITE Director Rita Katz said in a statement. “In recent weeks, about 10 Americans were indicted for attempts to act on behalf of the Islamic State, and there are many with every passing day that the group operates on Twitter.”

Some of the threat messages also included multiple reconstructed images, showing the destruction of the White House, the Statue of Liberty and other major American sites. While some tweets were specific threats toward U.S. soldiers, other pro-ISIS Twitter accounts used the hashtag to post earlier press releases of the ISIS, announcing the beheading video of American citizen James Foley and other incidents.

“This Twitter campaign, along with other ones like it in the past, shows the Islamic State's skill in forcing itself into Western conversations,” Katz said in the statement, adding that the “#WeWillBurnUSAgain” hashtag and the content posted by ISIS supporters are indication of “a unified and structured method of online mobilization by the Islamic State and its followers.”