The Islamic State in Iraq and Syria (ISIS) has threatened to attack Washington in a new video as Central Intelligence Agency director John Brennan said more terrorist attacks may be "in the pipeline".
Experts, while divided on whether a Paris-style attack is likely to happen on US soil, nevertheless agree that ISIS has the intention to strike international targets such as the United States.
"There is no question about their intentions and capabilities... the reach of the group is not regional but global, which means it can attack anywhere from Singapore to San Francisco," said Professor Yonah Alexander, director of the Inter-University Centre for Terrorism Studies.
An attack on the US in particular, Prof Alexander said, would be a significant part of ISIS propaganda, showing its supporters and other groups like Al-Qaeda that it is "able to bring the US to its knees".
Across the globe, officials and experts have warned of more ISIS attacks. Mr Brennan said on Monday that he did not consider the Paris attacks to be a "one-off event".
His remarks came as ISIS released a video threatening an attack on Washington. Posted on an ISIS propaganda website, the subtitled video features a fighter saying: "As we struck France in the centre of its abode in Paris, then we swear that we will strike America at its centre in Washington."
While the video's authenticity has not been verified, US cities reacted by beefing up law enforcement.
In Washington, the Metro Transit Police Department said it would increase police presence throughout the transit system. There would be "increased security sweeps in and around rail stations and other critical infrastructure" and "expanded random explosives screening at station entrances", according to a statement released on Monday.
In New York City, a new counter- terrorism team was also deployed.
Governors of at least 21 states across the US, meanwhile, said they would resist the settlement of Syrian migrants, citing security concerns, as one of the Paris attackers is suspected to have arrived in Europe among Syrian refugees.
Louisiana Governor and Republican presidential candidate Bobby Jindal said he had instructed state agencies to "take all available steps to stop the relocation of Syrian refugees" to his state.
But President Barack Obama told a news conference on Monday, after the G-20 summit in Turkey, that it was important not to "start equating the issue of refugees with the issue of terrorism".
He also announced plans for greater intelligence sharing with France, and said the US would intensify targeted air strikes and assistance to local ground forces in Syria and Iraq.
Rejecting calls to send large- scale US ground forces into Syria, he said: "We have the right strategy and we're going to see it through."
A day earlier, Deputy National Security Adviser Ben Rhodes said ISIS does not have the same capability in the US as it does in Europe, where thousands of fighters have travelled to Syria and returned to Europe - the numbers in the US are estimated to be less than 100.
But experts warn that it is not strictly a numbers game.
"Today we live in a virtual environment, the threat to the US could be due to copycats, lone wolves, and they (ISIS) do have terrorist cells already in the US," said Prof Alexander. He added that there is no way to completely prevent an attack, but "increased enforcement activities and improved intelligence sharing" could reduce risks.
More could also be done on a local level. "We really have to find some way to bring in civil society, the business community such as airlines, and the religious community... the battle can be fought on many different fronts," he said.