US security agencies, including the Federal Bureau of Investigation (FBI) and the Department of Homeland Security (DHS), have kicked off a massive security operation amid concerns that Pope Francis could be targeted by terrorist groups like the Islamic State (ISIS) and al Qaeda when he arrives in Washington on Tuesday to start his six-day visit to the United States.

Although there are no specific credible threats that exist during the Pope's visit, events the pope would attend in New York, Philadelphia and Washington would require massive amount of security, according to a bulletin distributed to law enforcement units in the US by the FBI and the DHS and obtained by CNN.

The bulletin said the activities involving Pope Francis have been designated as "national special security events."

ABC News recently reported that a 15-year-old boy was arrested for allegedly planning to do an ISIS-inspired attack on the pope in Philadelphia.

Law enforcers particularly expressed concern on potential lone wolf attackers "because of the difficulty in discovering such individuals or independent groups until they are operational."

The bulletin said the pope's visit may become a "a powerful motivator for groups or individuals with anti-Catholic or anti-Christian viewpoints."

About one million people are expected to attend Pope Francis' open-air Sunday mass in Philadelphia. Authorities have identified behavioural indicators of terrorists including "suspicious purchases of dual-use items that could be used to construct an explosive device, unusual or prolonged interest in motorcade movement dynamics and security, and discreet use of cameras or video recorders, sketching, or note-taking."

The bulletin was issued to "aid law enforcement and first responders in identifying and mitigating threats."

Homeland Security Secretary Jeh Johnson noted the difficulty of stopping lone-wolf home-grown terrorists.

"It is, for the most part, smaller scale attacks, but, in many respects, harder to detect, because it involves the so-called lone wolf who could strike with little or no notice in the homeland," he said.

Federal authorities are mobilising large security operations to protect the pope.

The pope is expected to arrive at Joint Base Andrews in Washington D.C. at around 4 p.m. on Tuesday, Sept. 22.

He is scheduled to visit the White House and meet President Barack Obama the following day. In the morning of the same day, there will be a papal parade along the Ellipse and the National Mall and a Midday Prayer with US bishops at Saint Matthew's Cathedral in D.C. In the afternoon, the pope will preside the Junipero Serra Canonisation Mass at the Basilica of the National Shrine of the Immaculate Conception.

On Thursday, Sept. 24, the pope will arrive at the Capitol and deliver a speech to a joint session of Congress. He is also scheduled to visit St. Patrick's Catholic Church in D.C. and Catholic Charities of the Archdiocese of Washington.

In the afternoon, he will depart for New York from Joint Base Andrews. Following his arrival at John F. Kennedy International Airport in New York, he will hold an evening prayer at St. Patrick's Cathedral.

On Friday, Pope Francis will attend a session of the United Nations General-Assembly and a multi-religious service at 9/11 Memorial and Museum at the World Trade Center.

In the afternoon, he will visit the Our Lady Queen of Angels School in East Harlem, hold a papal motorcade through Central Park, and officiate in a Madison Square Garden Mass.

On Saturday, Pope Francis will depart for Philadelphia where he will hold a Mass at Cathedral Basilica of Sts. Peter and Paul. He is also scheduled to visit the Independence Mall and the Festival of Families at Benjamin Franklin Parkway before attending a prayer vigil with World Meeting of Families.

On, Sunday, the pope will meet with American bishops at St. Martin's Chapel, St. Charles Borromeo Seminary, visit a correctional facility, and preside a mass for World Meeting of Families before departing for Rome.

The US Secret Service is in charge of coordinating the massive operation on counterterrorism operations, crowd management, crisis response and air and vehicle traffic control along with the FBI, Capitol Police, Coast Guard, Pentagon. Federal Emergency Management Agency and local police.

On Friday, while Pope Francis is in New York, the US will also welcome another major world figure—Chinese President Xi Jinping, whose visit will pose an additional security challenge to the US.

"They're standing up one of the largest security programmes for an individual in US history over three cities," said Jonathan Wackrow, a former Secret Service agent who assisted in security preparations for Obama's 2013 inauguration. "The level of protection afforded to Pope Francis is equal to or exceeding that provided to the president of the United States."