Two American women inspired by Al-Qaeda and extremists in Syria were arrested in New York on Thursday, charged with planning to build a bomb and attack the United States, prosecutors said.

Former roommates Noelle Velentzas, 28, and Asia Siddiqui, 31, risk life behind bars if convicted of conspiracy to use a weapon of mass destruction in the United States.

The pair from Queens were shadowed for nearly two years by an undercover FBI agent and appeared before a magistrate at the federal district court in Brooklyn on Thursday.

Siddiqui was "in possession of multiple propane gas tanks, as well as instructions for how to transform propane tanks into explosive devices," court papers state.

The women had not decided on a target, although documents show that married Velentzas mentioned a New York police funeral as an "attractive potential target."

She also said they should be referred to as "citizens" of the Islamic State (IS) extremist group fighting in Iraq and Syria and that "there were more opportunities of 'pleasing Allah' in the United States" than traveling overseas to fight, prosecutors said.

The pair allegedly studied Al-Qaeda's English-language magazine "Inspire" and "The Anarchist Cookbook" for instructions on how to build a bomb, and they had started to acquire some ingredients.

Siddiqui wrote a poem in an Al-Qaeda publication as early as 2009 and Velentzas called Osama bin Laden one of her heroes, allegedly keeping a picture of the Al-Qaeda mastermind on her mobile phone.

- Praise for 9/11 attacks -

They communicated with Al-Qaeda in the Arabian Peninsula, which is based in Yemen, and watched videos of beheadings carried out by IS extremists in Syria, the complaint said.

In a self-proclaimed effort to "make history," prosecutors said the pair researched previous attacks and acquired some components of bombs used in the 1993 World Trade Center bombing, the 1995 Oklahoma City bombing and the 2013 Boston Marathon bombing.

As with the ethnic Chechen brothers accused of bombing the Boston Marathon in April 2012, the women allegedly had online jihadist instructions on how to make explosive devices.

"We remain firm in our resolve to hold accountable anyone who would seek to terrorize the American people," said Loretta Lynch, Attorney General nominee and current US attorney for the eastern district of New York.

The two women bring to six the number of people her office has charged in connection with IS-linked terror plots in recent weeks.

Siddiqui was allegedly close to Pakistani-American citizen Samir Khan, who went on to edit Al-Qaeda's English-language "Inspire" magazine and was killed in a US drone strike in 2011.

Velentzas praised the 9/11 attacks and suggested they learn the science behind explosives to avoid the fate of Faisal Shahzad, the Pakistan-American who attempted to blow up Times Square in 2010.

He was captured and jailed for life.

- Suspected undercover agent -

She once allegedly pulled a knife from her bra and demonstrated to Siddiqui and the agent how she would stab someone if attacked.

The two women discussed how to avoid detection, and browsing history suggests that Velentzas at one point suspected the undercover officer.

She was also Facebook friends with former US Air Force mechanic Tairod Pugh, 47, who pleaded not guilty in a separate IS alleged plot before a Brooklyn court last month, the complaint said.

Thursday's arrests are the latest in a string of alleged plots inspired by Islamic extremists.

US intelligence officials warned in February that more than 20,000 volunteers from around the world, including more than 150 Americans, had gone to Syria to link up with extremists.

A Texan-born American deported from Pakistan was separately due to appear in a New York court charged with conspiracy to provide material support to terrorists, prosecutors said.

Muhanad Mahmoud al-Farekh allegedly traveled to Pakistan in 2007 while enrolled as a university student in Canada, to fight jihad and join Al-Qaeda.