US officials have charged 14 people with providing money, personnel and services to the Somali militant group al-Shabab.
The charges stem from four separate indictments in the US states of Minnesota, Alabama and California.
Concerns about al-Shabab have grown after the group carried out a bomb attack in Uganda.
On Wednesday, a man was charged with attempting to support al-Qaeda and al-Shabab.
Prosecutors said Shaker Masri of Chicago had sought to travel to Somalia to join al-Shabab, which the US and UK consider a terrorist group.
The indictments unsealed on Thursday make reference to four specific individuals as well as a group of 10 men charged with providing resources to al-Shabab, which has close ties to al-Qaeda.
In two separate indictments, prosecutors charged Shafik Hammami, a former resident of the US state of Alabama, and Jehad Serwan Mostafa, formerly of California, with providing material support to al-Shabab.
In the District of Minnesota, prosecutors unsealed an indictment charging Amina Farah Ali and Hawo Mahamed Hassan with providing funds to the Somalia-based militant group. Both defendants, who are naturalised citizens were arrested on Thursday.
The two are accused of having raised money in Somali communities in the US and Canada, telling people the funds would help the poor and needy in Somalia.
Prosecutors in Minnesota also charged 10 men with terrorism offences for leaving the United States to join al-Shabab - seven of them have been previously charged.
US Attorney General Eric Holder said in a news conference a total of 19 defendants had now been charged in connection with the ongoing investigation.
Mr Holder said nine of the suspects had been arrested, with five pleading guilty.
"The indictments unsealed today shed further light on a deadly pipeline that has routed funding and fighters to the al-Shabab terror organisation from cities across the United States," Mr Holder said.
He said the latest charges reflected a growing trend in the US - one which leaders of Muslim communities were trying to help the authorities address.
"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
"It's a disturbing trend that we have been intensely investigating in recent years and will continue to investigate and root out. But we must also work to prevent this type of radicalisation from ever taking hold," he added.
Officials believe up to 50 US citizens may be fighting with al-Shabab in Somalia, according to NBC News. Some of these individuals are believed to be from Minneapolis, Minnesota and Columbus, Ohio.
In 2008, the US government designated al-Shabab, which seeks to establish an Islamic state within Somalia, a foreign terrorist organisation.
Al-Shabab is believed to have recruited Somali-Americans to take part in suicide bombings in Somalia, and US officials are now growing concerned about the potential for an attack within the US.
Al-Shabab - which has been fighting the Somali government since 2006 - wants to establish an Islamist administration and is said to have thousands of fighters.
Last month, the group said it had been responsible for two bomb blasts in the Ugandan capital, Kampala, which killed 76 people during the World Cup.