"Nine have been arrested in the US or overseas, five of whom have already pleaded guilty. Ten of the charged defendants are not in custody and are believed to be overseas."
US prosecutors revealed indictments against three individuals charged with supporting and providing resources to the Somali anti-government fighters, saying the defendants tried to raise funds using a bogus charity as a front.
Two Somalia-born US citizens living in Minnesota were accused of raising money and sending it to al-Shabab and lying to FBI investigators, according to one indictment unsealed in a Minnesota court.
Amina Farah Ali and Hawo Mohamed Hassan were charged in a 16-count indictment that detailed their efforts to raise money for al-Shabab by going door-to-door in the Somali communities in Minnesota and elsewhere in the United States and Canada, the indictment said.
They also held teleconferences to raise funds, sometimes saying it would be used to help the poor and needy in Somalia.
Ali was accused of sending about $8,600 to al-Shabab, according to the indictment.
US prosecutors also unsealed a three-count indictment from 2009 that charged a young man, Omar Shafik Hammami, who grew up in Alabama but left for Somalia, with providing material support and resources to al-Shabab.
The Obama administration has been increasingly concerned about al-Shabab, particularly after the group successfully carried out a suicide bombing attack in Uganda killing more than 70 people, including one American.
That was the first successful strike by the group outside Somalia. US intelligence agencies have also warned of growing links between al-Shabab in the partially lawless country and al-Qaeda's network in East Africa.
The unsealing of the indictments follows the arrest of two other American men in the last few weeks in separate cases on charges they were trying to travel to Somalia to aid al-Shabab. One was arrested in Virginia and another in Chicago before they could leave for the Africa country.
Holder said: "As demonstrated by the charges unsealed today we are seeing an increasing number of individuals including US citizens who have become captivated by extremist ideology and have taken steps to carry out terrorist objectives either at home or abroad.
"This is a very disturbing trend … that we will continue to investigate and root out. But we must also work to prevent this type of radicalisation from ever taking hold."
Roslind Jordan, Al Jazeera's correspondent in Washington DC, said that there would now be added emphasis on arresting those indicted who are still at large.
"Some of the people apparently indicted have not been brought into custody, so certainly there is going to be an international push to try to find the suspects and bring them to the US for prosecution.
"This is part of the Obama administration's overall strategy not to deal with the so-called war on terror in a military sense but rather to deal with people who would be accused of committing acts considered terrorist acts under US law through the federal courts."