The F.B.I. has increased its surveillance of Syrians inside the United States in response to concerns that a military strike against the government of President Bashar al-Assad could lead to terrorist attacks here or against American allies and interests abroad, according to current and former senior United States officials.

The government has also taken the unusual step of warning federal agencies and private companies that American military action in Syria could spur cyberattacks, the officials said. There were no such alerts before previous military operations, like the one against Libya in 2011.

The authorities are particularly concerned because Iran — one of Mr. Assad’s closest allies — has said there will be reprisals against Israel if the United States attacks Syria. The Iranians have also shown a willingness to sponsor terrorist attacks on American targets, according to the officials, speaking on the condition of anonymity to discuss a continuing operation.

“They’re not starting from scratch — 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 senior American official said.