The FBI is upping its surveillance of Syrians within US borders, for a fear of reprisal attacks that may follow an imminent American strike against Bashar Assad, reports in US media say. Homeland Security is also worried about retaliatory cyber-attacks.

The Federal Bureau of Investigations has sent out a classified bulletin warning about potential fallout from the Syrian conflict to all law enforcement agencies, and hundreds of Syrians may face interviews with agents in the coming days, according to unnamed senior officials who spoke to the New York Times.

All sources connected with any Syrians, have been told to look out for plans or even “talk” of a retaliatory strike on US targets, whether inside its borders, or abroad. Those under investigation will presumably include US citizens of Syrian origin.

“The field offices know what they have in terms of sources and investigations, but this is a directive for them to redouble their efforts and check their traps,” one highly-placed official told NYT.

The practice of questioning people on the basis of their ethnic links is a long-standing policy, US Agents asked Iraqi-Americans to report on suspicious behavior ahead of Operation Iraqi Freedom, and Libyan-Americans before the air strikes that helped topple Muammar Gaddafi two years ago.

But not everyone is convinced by the profiling policy.

“We didn’t think the return on investment was especially high given the time and resources required to contact that many people,” James W. McJunkin, a former leading FBI counterterrorism official, said of the previous operations to uncover terrorist cells and ‘lone wolves’.

For the Department of Homeland Security, one new aspect of the Syrian operation is the increased focus in defending US authorities and companies against cyber-warfare.

Over the past year, the Syrian Electronic Army, a pro-Assad hacking group, has targeted several high-profile but poorly defended US companies, mostly causing nuisance and embarrassment. This week the New York Times website was blocked by the group, while it previously hijacked the Twitter feed of news agency AP, and claimed that Barack Obama was wounded in an explosion, setting off a brief worldwide panic.

The group, which is thought to receive support from the government of Bashar Assad, has promised “many surprises” if the US executes a strike on Syria, for the alleged use of chemical weapons against civilians.

"Military intervention in Syria has many consequences and will affect the whole world,” a group spokesman told BBC.

But both in the fields of virtual warfare and physical reprisals, the US is said to be focusing as much on Syria’s powerful backer Iran, as on the country itself. In the past, Iranians have been accused of carrying out a spate of embassy attacks aimed at Israel and the US, and have threatened a retaliatory response against any direct intervention in the Syrian civil conflict.

"The fact that the Americans believe that military intervention will be limited to within Syria's borders is an illusion; it will provoke reactions beyond that country," Mohammad Ali Jafari, commander of the influential Revolutionary Guard told ISNA news agency on Saturday.

