Colombian forces have killed at least 20 Marxist FARC guerrillas in an air and ground raid near the border with Ecuador, an army general said, the deadliest strike against rebels since a peace process started.

Despite talks to end 50 years of war, Colombia's government has vowed to keep up military operations, even while the Revolutionary Armed Forces of Colombia, or FARC, called a two-month ceasefire as the two sides try to thrash out a deal.

Colombian President Juan Manuel Santos hopes a decade of US-backed blows against the FARC has left the group sufficiently weakened to seek an end to the war.

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In the most deadly attack since the two sides started negotiations in mid-October, an airstrike followed by a ground assault on three FARC camps in the southwestern Narino department killed at least 20 rebels. "We're talking about more than 20 dead, but the figure could be higher," General Leonardo Barrero, head of the Joint Command Southwest, said.

Mr Santos said peace talks, due to resume this week, should not drag on for too long. Both sides said negotiations were going as expected.