U.S. military, law enforcement and intelligence personnel have been warned that they could be targeted by the Islamic State of Iraq and Syria (ISIS).

A new security bulletin issued by the Department of Homeland Security and the FBI advises that U.S. intelligence has seen ISIS figures overseas calling for individuals in the U.S. to attack above mentioned groups.

Federal officials warned U.S. personnel to limit their online postings, so as to not give out any information that would leave them vulnerable to attacks.

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The FBI said there is no information available about a specific attack or imminent threat.

Concerns about domestic terrorist attacks have been raised in the last few months after a pair of apparent lone-wolf attacks in Canada resulted in the killing of Canadian soldiers. In one, a 22-year-old described as an ISIS-inspired terrorist rammed a pair of Canadian soldiers with his car, killing one. He was killed during the getaway. In the second, another 22-year-old radicalized online shot and killed a Canadian soldier standing guard at the country's national war memorial. The gunman was later killed in a shootout inside the parliament building in Ottawa.

In neither case have authorities said the killers had direct contact with any ISIS members.

Thousands of foreign fighters are believed to be fighting for ISIS and other jihadist groups in Syria and Iraq. As recently as the summer, U.S. officials had identified a handful of Americans linked with the radical group, but sources told CBS News' Bob Orr the actual number was likely in the dozens.

Terror analysts say those fighters pose the greatest threat to the United States because of their ability to travel freely and blend in. Many are recruited through a powerful online media campaign.

"I am your brother in Islam here in Syria. We have safety here for your family and children," said a Western jihadist on video, urging potential ISIS recruits to come join the fight in Syria.

It's all part of a high-tech propaganda machine ISIS has developed to reach out to militants in Europe, Canada and the United States.

The terror group now has its own multilingual media arm, Al Hayat, which is behind the creation and distribution of glossy magazines and highly produced slick videos. ISIS even uses drones and GoPros to appeal to the Western eye.

Secretary of State John Kerry said in September that stopping the flow of foreign fighters joining ISIS will be far more important than airstrikes in the mission to stamp out the extremist group.

ISIS has repeatedly called for attacks on American targets since a U.S.-led coalition began bombing extremist targets in Syria and Iraq this year.

The Sunni militant group has also overrun a large part of Iraq's Anbar and Ninevah provinces and now controls about one-third of both Iraq and Syria.

Over the weekend, at least 30 ISIS targets around its de facto capital of Raqqa in Syria were hit by the U.S.-led coalition.

Raqqa has been the site of heavy bombardment by Syria government forces as well. Activists say 95 people, including many civilians, were killed in strikes there by Assad government air force bombings last week.

The American-led coalition began targeting ISIS in Syria in September, expanding an aerial campaign already hitting the extremist group in Iraq.