Federal officials are promising nearly $1 billion for the extension of the MBTA's Green Line to Somerville and Medford.

Therese McMillan, acting administrator for the Federal Transit Administration, announced the funding in a letter to U.S. Sen. Tim Johnson, chairman of the Senate committee on Banking, Housing and Urban Affairs.

McMillan said the $996 million was projected to pay for about 43 percent of the project, with the state providing the rest of the money through a combination of capital and operating funds.

MBTA General Manager Beverly Scott welcomed the announcement Tuesday, calling the Green Line extension one of the best transit projects in the country.

The plan calls for extending the Green Line along two branches from a relocated Lechmere Station in Cambridge to Union Square in Somerville and College Avenue in Medford.