A 30-year-old woman from Philadelphia was charged Friday with attempting to join extremists in Syria, the second Syria-linked terror case involving US women announced by prosecutors in days.

Keonna Thomas, 30, was charged with attempting to provide material support to a terrorist organization after allegedly posting more than a year of tweets in support of the Islamic State (IS) group.

If convicted, she faces up to 15 years in prison.

After posting the stream of alarming tweets from August 2013 to December 2014, she applied for a US passport and intended to travel to Syria through Spain and Turkey, prosecutors said.

Having applied for the passport in February, she emailed a radical cleric in Jamaica saying she had deactivated her Twitter account until leaving for Syria to avoid attention, they added.

Among her disturbing tweets was one last December. Next to the picture of an armed child she wrote: "I wouldn't be pleased till I became soldier of the Islamic State."

Under FBI surveillance, she allegedly emailed an IS extremist based in the group's stronghold of Raqqa in northern Syria.

In February she allegedly wrote to him that it "would be amazing" to carry out a suicide attack, adding that "a girl can only wish," court papers show.

She allegedly bought a visa for Turkey and searched the Internet for indirect travel routes to the country, the most common transit point into Syria for Western recruits to IS.

Prosecutors say she bought an airline ticket from Philadelphia to Barcelona intending to fly on March 29.

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

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.

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.