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The contract, worth up to $360m (£224.6m) over the 10 years, is to provide Internet Protocol (IP) and Asynchronous Transfer Mode (ATM) services over international private lines and satellite links to State Department facilities worldwide.
The agreement, known as Spectrum, also makes WorldCom a preferred provider for future State Department communication contracts.
WorldCom has had an off-and-on relationship with the US federal government this year.
In July, the company lost a $1.7bn contract with the US Federal Aviation Authority to run its telecommunications infrastructure.
The US Securities and Exchange Commission investigation into WorldCom's finances led to speculation that the company would be barred from doing business with the US government by the General Services Administration (GSA), which regulates many government contracts.
That uncertainty came to an end in November when the GSA renewed a contract with WorldCom to provide telecommunications services. The GSA barred two former executives of WorldCom from doing business with the government, but not the company itself.