President Juan Manuel Santos on Monday welcomed the appointment of a US special envoy to Colombia's talks with FARC rebels as a strong signal of Washington's interest in a peace agreement.

The United States announced last week it has named Bernard Aronson, a former assistant secretary of state for inter-American affairs, as a special envoy to the talks, which have been underway in Havana for more than two years.

"The United States doesn't do that every day," Santos said in a television interview. "They very rarely take a political decision to name someone as a special envoy to solve a problem."

"It's a signal that says the United States is really interested in finding a solution to the problem," he added.

Aronson, who came out of retirement to take the assignment, is known for his role in resolving the conflicts in El Salvador and Nicaragua in the late 1980s and early 1990s.

The United States, which sees Colombia as one of its closest allies in the region, has been deeply involved in Bogota's counter-drug and counter-insurgency efforts over the past 15 years under its "Plan Colombia" program of military and economic cooperation.

Colombia's civil conflict, the oldest in Latin America, has claimed more than 220,000 lives over the past half century.

Santos's government has been in peace talks since November 2012 with the Revolutionary Armed Forces of Colombia (FARC), the country's largest rebel group with an estimated 8,000 fighters.

His government has also held preliminary talks with the country's other guerrilla group, the National Liberation Army, or ELN by its Spanish acronym.

The FARC has observed a unilateral ceasefire since December 20, but Santos has refused to reciprocate, insisting that a comprehensive peace agreement be reached first.