The FARC are the biggest rebel group in Colombia

Manuel Marulanda, of the left-wing Revolutionary Armed Forces of Colombia (FARC), said it was time that the rebels and the army began serious negotiations.

The guerrilla leader, known as "Tirofijo" or Sure-shot, made the call in an open letter to army colonels.

He said that through a civilised exchange of opinions, both sides could resolve their differences and work toward a definitive solution to the political, economic and social causes of the conflict.

A similar letter to the army high command five months ago received no response.

Peace hopes dashed

The FARC is the largest guerrilla group in Colombia and is thought to have about 18,000 members.

The last attempt to engage the FARC in a peace process collapsed in February 2002, when President Andres Pastrana broke off three years of talks.

He was succeeded in office by Alvaro Uribe, whose presidency has ushered in a hardline approach to rebel groups.

In August, the FARC and the smaller National Liberation Army (ELN) issued a rare joint statement ruling out negotiations with President Uribe, whom they described as an enemy of peace.