Colombia's FARC rebels killed four soldiers and wounded six by setting a landmine that exploded when an army helicopter landed on it, officials said.

The attack Monday came as the Marxist group steps up operations as peace talks under way since late 2012 start to unravel.

On Tuesday, the FARC issued another call for a bilateral ceasefire, but rhe government has always refused, saying the rebels such truces as a chance to regroup and re-arm.

The helicopter was providing air support for ground troops protecting a major oil pipeline in Norte de Santander department, which borders Venezuela.

The Black Hawk chopper hit a mine as it landed, then exploded, General Luis Maldonado of the army said. He said the rebels had placed the mine.

"Four brave soldiers died today in Teorama," Maldonado said.

Another FARC attack on an oil pipeline on Sunday caused a spill in Narino department in the southwest.

That was the second time in a week the rebels reportedly attacked government forces and a pipeline, continuing a series of attacks since FARC suspended a unilateral ceasefire in May.

Last week, rebels sabotaged the country's Cano Limon-Covenas pipeline -- the one involved in Monday's incident with the helicopter -- then attacked an army patrol in the south, killing four soldiers and wounding four others.

The escalating attacks against infrastructure and government forces come as talks in Cuba to end the country's 51-year-old conflict show signs of fraying.

Although the talks have advanced on a number of major issues, a final accord has yet to surface.

In April, President Juan Manuel Santos resumed airstrikes against FARC after the killing of several soldiers in a guerilla ambush.

More than 200,000 people have been killed in Colombia's conflict since FARC was launched in 1964.