The deal between the world's top economy and Asia's fourth largest will boost trade by billions of dollars and create tens of thousands of jobs, the two sides say, making it one the biggest deals of its kind.

"The US-Korea agreement is a landmark deal with an important ally," US Trade Representative Ron Kirk said in a statement hailing the accord as the most significant US free trade pact in 20 years.

"Starting today, Korea's doors are wide open for Made-In-America exports that will support well-paying jobs here at home," Kirk said.

The deal has provoked outbursts of violence in South Korea's parliament and street protests, mostly by farmers, including a small demonstration in the capital on Thursday.

The leader of the main opposition party, Han Myung-sook, this week backed down from a vow to repeal the deal altogether, saying she only wanted parts of it renegotiated.

Experts doubt major changes to the deal will be enacted.

U.S. President Barack Obama and South Korean President Lee Myung-bak spoke by telephone and praised the pact as symbolic of a strengthening of their alliance, the presidential Blue House in Seoul said in a statement.

"The South Korea-U.S. FTA is a high-standard treaty that will become a good model of global free trade," Lee was quoted as telling Obama.