WASHINGTON (Reuters) - The United States and Colombia have agreed on a plan to address concerns over labor rights and anti-union violence in the South American country that have blocked a free trade pact for more than four years, a U.S. private sector source said on Wednesday.

"I hear the administration reached a deal with Colombia and will begin making calls this morning to stakeholders," the source said, speaking on condition he not be identified.

A spokeswoman for the U.S. Trade Representative's office said she had no details to share at this time.

On Tuesday, a U.S. source close to the talks with Colombia had said an announcement could come as early as Wednesday.

Rising expectations of an announcement came as Colombian President Juan Manuel Santos was in the United States this week for a United Nations meeting.

A deal would clear the way for President Barack Obama to submit the trade agreement to Congress for a vote. On Tuesday, U.S. Trade Representative Ron Kirk told a congressional panel Obama wanted to submit the deal this year.

A deal also would jump-start action on a free trade pact with South Korea, which key U.S. lawmakers have put on hold until Obama sends both the Colombia agreement and a separate deal with Panama to Congress.

In a speech at Brown University in Providence, Rhode Island, on Tuesday, Santos did not mention the labor talks or the pending free trade agreement, but urged the United States to step up its economic involvement in Latin America.

The administration of former President George W. Bush negotiated the trade deal with Colombia, which the countries signed in November 2006.

But Democrats, who won control of Congress in elections that same month, objected strongly to the pact on the grounds that then-President Alvaro Uribe had not taken strong enough steps to protect workers' rights and killings of union leaders by paramilitary and other right-wing groups.

(Reporting by Doug Palmer; editing by Vicki Allen)