WASHINGTON (Reuters) - U.S. immigration agents have arrested more than 1,000 suspects on charges including attempted murder and witness tampering, in a nationwide operation aimed at international gangs, officials said on Monday.

The five-week operation headed by U.S. Immigration and Customs Enforcement (ICE) agents, dubbed Project Shadowfire, targeted criminal organizations linked to drug trafficking, murder and racketeering, the agency said in a statement.

Of the 1,133 arrested, 915 were cross-border gang members and associates. Just over 1,000 were charged with criminal offenses and the rest face immigration charges, it said.

A majority of those arrested had links to notorious groups such as MS-13, Sureños, Norteños, Bloods and several prison-based gangs. The operation was centered on the Los Angeles; San Juan, Puerto Rico; Atlanta; San Francisco; Houston; and El Paso, Texas areas.

While most of those detained were Americans, 239 were nationals from Mexico, Spain, El Salvador, China, Jamaica, Guatemala, Honduras, Philippines, Belize and other countries.

The special agents seized 150 firearms, more than 20 kg of narcotics and more than $70,000 in U.S currency.

Immigration and Customs Enforcement has staged similar operations targeting gangs since 2005. Although ICE is an immigration authority, it routinely participates in operations involving U.S. citizens and non-citizens due to its expertise on international crime groups.