US counterterrorism authorities on Friday arrested a Pennsylvania teen on charges of aiding the Islamic State in Iraq and Syria (Isis) armed group after finding an arsenal of military weaponry in his backpack.

Jalil Ibn Ameer Aziz, 19, was charged with “conspiring and attempting to provide material support to Isis, the FBI in the Pennsylvania state capital of Philadelphia said in a press release.

“A prior physical search of Aziz’s residence also revealed a tactical-style backpack in his closet, which contained high-capacity weapons magazines, ammunition, a knife, and other survival items,” Assistant Attorney General for National Security John P. Carlin said in the release. “The National Security Division’s highest priority is counterterrorism, and we will remain vigilant in our efforts to hold accountable any and all those who seek to provide material support to terrorists, and to disrupt potential attacks in the United States before they happen.”

FBI agents searched Mr Ameer Aziz’s backpack and found “five load M4-style high-capacity magazines, a modified kitchen knife, a thumb drive, medication and a balaclava.”

The charges allege that Mr Ameer Aziz used 57 Twitter accounts “to advocate violence against the United States and its citizens, to disseminate ISIL propaganda and espouse pro-ISIL views.” The accounts were also allegedly used to aid in communication between Isis operatives in the Middle East and people seeking to travel to Isis-held territory as foreign combatants.

“Aziz allegedly acted as an intermediary between a person in Turkey and several well-known” Isis members, the FBI said.

Aziz also allegedly posted the name of 100 people believed to be US military and called for violence against them.

“The FBI’s Joint Terrorism Task Force (Jttf) will move with speed to detect and disrupt those who are involved in facilitation or operational planning in support of a terrorist organization. There is no difference between the two, and we will aggressively pursue every threat in order to protect our communities,” said Special Agent in Charge Sweeney.

Mr Ameer Aziz’s case is still under investigation by the Task Force and authorities have yet to set his trial date.

His arrest on allegations of aiding terrorists comes amid fears from the US Muslim community of a backlash over a series of Isis-inspired attacks nationally. Last week, Republican presidential frontrunner Donald Trump to bar Muslims from entering the country following a deadly attack in San Bernardino, California believed to have been inspired Isis.