US law enforcement and intelligence agencies are concerned about copycat vehicle-ramming attacks following the New Year's Day attack in New Orleans by a US Army veteran, according to an intelligence bulletin.
The bulletin was issued a day after the FBI said Shamsud-Din Jabbar, a Texas native, was "100% inspired" by the Islamic State militant group to drive a truck into New Year's Day revelers in New Orleans, killing at least 14 people and injuring dozens of others.
He was subsequently killed in a shootout with police.
The FBI, Department of Homeland Security and the US National Counterterrorism Center "are concerned about possible copycat or retaliatory attacks," said the intelligence bulletin published by the three agencies and reviewed by Reuters.
Such attacks "are likely to remain attractive for aspiring attackers given vehicles' ease of acquisition and the low skill threshold necessary to conduct an attack," said the bulletin issued to US law enforcement agencies.
It urged law enforcement personnel and private security firms to be aware that in many previous cases attackers who rammed vehicles into crowds were armed and continued their attacks with guns or edged weapons.
The 1 January incident in the packed French Quarter of New Orleans was the seventh attack in the United States since 2001 that was inspired by a foreign extremist organisation, the bulletin said.
The use of "edged weapons" and firearms has been more common in such attacks but vehicles could present a growing threat, it said.
Meanwhile authorities in several US cities said they have now boosted security, including at Trump Tower and Times Square in New York City, adding that there were no immediate threats.
In Washington, police said they have increased their presence as the capital prepares to host three major events this month.
The events include Congress' certification of US President-elect Donald Trump's presidential election win on 6 January, the state funeral for former President Jimmy Carter on 9 January and Mr Trump's inauguration on 20 January.
