Twitter is being sued by the widow of a US citizen killed in Jordan who has accused the social media company of giving a voice to the so-called Islamic State, adding to the pressure to crack down on online propaganda linked to terrorism.

Tamara Fields, a Florida woman whose husband Lloyd died in a 9 November attack on a police training centre in Amman, said Twitter knowingly let the militant Islamist group use its network to spread propaganda, raise money and attract recruits.

Lawyers specialising in terrorism said Ms Fields faces an uphill battle, though the case could lead to more calls for social media companies such as Twitter and Facebook to take down posts associated with terrorist groups.

In her complaint filed on Wednesday, Ms Fields said Twitter had until recently given IS, also known as ISIS, an "unfettered" ability to maintain official Twitter accounts.

"Without Twitter, the explosive growth of ISIS over the last few years into the most-feared terrorist group in the world would not have been possible," according to the complaint, which was filed in the federal court in Oakland, California.

Ms Fields wants Twitter to pay her triple damages for violating the US federal Anti-Terrorism Act by having provided material support to terrorists.

Her lawyer said he believes it is the first case in which a social media company is accused of violating that law.

"While we believe the lawsuit is without merit, we are deeply saddened to hear of this family's terrible loss," Twitter said in a statement about the civil lawsuit.

"Violent threats and the promotion of terrorism deserve no place on Twitter and, like other social networks, our rules make that clear."

Last Friday, US President Barack Obama’s administration set up a task force to crack down on extremist groups using the internet to advance either goals, find recruits and plan attacks such as recent killings in Paris and San Bernardino, California.

Senior national security officials also met with technology executives in Silicon Valley last week to discuss what more could be done to counter Islamist militants.