Security has been stepped up for employees of a German magazine and for members of a small far-right party after a known terrorist called for them to be killed in a video online, officials said Tuesday.

The video surfaced May 18 on YouTube following street clashes between an ultraconservative Muslim group and the far-right Pro NRW party in Bonn and Solingen, in western Germany.

Three security officials, speaking on condition of anonymity because of the sensitivity of the issue, said they have no specific evidence that attacks are being planned in the wake of the video. But they said they worry it could inspire a lone-wolf type attacker like Arid Uka, an Islamic extremist who killed two U.S. airmen at Frankfurt airport last year after being spurred to action in part by a YouTube video preaching jihad, or holy war.

"The danger is very real," said one official.

In the Bonn skirmish on May 5, some 110 Salafist protesters were arrested after they started attacking police with stones and bottles. The police were trying to keep them away from about 30 Pro NRW supporters. Twenty-nine police officers were injured; two were hospitalized with stab wounds.

The rally by the marginal anti-immigration Pro NRW party was one of several it held around North-Rhine Westphalia, Germany's most populous state, ahead of a May 13 state election. Pro NRW won only 1.5 percent of the vote, far short of the score needed to win seats in the state legislature.

The Salafist protesters were upset by Pro NRW's anti-Islam signs, particularly those showing the cartoons depicting the Prophet Muhammad that provoked outrage in 2005 when they were first printed in a Danish newspaper. Strict Muslims oppose any depiction of their prophet.

In the video, Bonn-born Yassin Chouka, who is believed to be with the Islamic Movement of Uzbekistan in the Afghanistan-Pakistan border region, calls for Muslims to kill Pro NRW members.

Speaking in fluent German, Chouka urges the Salafists to move away from street confrontations where the risk of being arrested is too great. He instead tells them to target Pro NRW members at their homes and workplaces, "at best under cover of darkness."

"We should kill all the members of Pro NRW," he said.

He also said employees of Der Spiegel -- which has printed photos of the demonstrations showing the Prophet Muhammad signs -- and others in the media should be targeted.

"Der Spiegel, which is linked to the Jews, and other German media have insulted the prophet," he said in the seven-minute audio message, played with a picture of the street fights in the background.

As a result of the video, authorities have taken security measures at Der Spiegel's Hamburg headquarters and also in Berlin, the officials said without giving more specific details. In addition, at least one of the Pro NRW members who has been identified by name in pictures has been assigned a security detail.

Particularly alarming was the call in the video to track down individuals -- an escalation from general non-specific calls for attacks by jihadi groups online.

"That is something we have not yet seen," said another of the officials.

Germany's intelligence and security agencies have been increasingly monitoring the actions of the Salafists, who number some 4,000 in Germany, in recent months.

Overall, the officials said some 130 radical Muslims in Germany are known to authorities as being potentially violent, of whom 24 are part of the Salafist movement.

"Of bigger concern are the others -- the individuals who have not yet been identified," said the first official. "Young people who decide to do something, either alone or with others, with simple means."