(Adds quotes, background on peace process, recent attacks, paragraphs 2, 4-5, 7-12)
By Peter Murphy
BOGOTA, July 29 (Reuters) - Colombia's President Juan Manuel Santos warned on Tuesday that continuing attacks perpetrated by leftist FARC guerrillas, many of which target infrastructure, could bring an end to peace negotiations with the government.
The threat was made weeks after Santos' successful re-election campaign, which made much of agreements he had reached with the rebels on more than half the agenda for peace talks that he initiated in late 2012 to end a half-century of war.
"What we are saying to them is, keep this up and you are playing with fire and this (peace) process can end," Santos said at an industrial event.
He referred to infrastructure attacks, including a major one this week on an electricity pylon that cut power to Buenaventura, with more than a quarter of a million people the largest city on Colombia's Pacific coast .
"It's something demented ... They are digging their own political grave because this is exactly what makes people reject them," Santos said, referring to another recent attack on an aqueduct that left several towns without water.
The FARC have been fighting successive governments for 50 years in a conflict that has killed around 220,000 people and which mushroomed out of a peasant movement seeking land reform.
The rebels have refused to declare a ceasefire during the peace talks unless the government reciprocates, something Santos has ruled out. The United States and the European Union have declared the FARC a terrorist organization.
The peace talks, which are being held in Havana, appear to have gone further than previous attempts by other governments. There have been no unscheduled or acrimonious recesses in the 21 months since they began.
They have so far reached agreement on three of the five points on the agenda - land reform, the rebels' participation in politics, and withdrawal from the drugs trade. A framework for ending the conflict and victim compensation are the remaining items to be agreed upon.
Achieving peace with the FARC and their smaller counterpart the ELN, or National Liberation Army, was the dominant issue in the presidential election that Santos won in a second round on June 15. His second term begins on Aug. 7.
Despite the progress Santos demonstrated, the election showed the strength of skepticism about the peace talks. Santos' rival Oscar Ivan Zuluaga, who spoke of ending the talks to pursue the rebels on the battlefield, led the race until close to the vote.
Separately on Tuesday, the ELN detonated four small explosive devices around the capital city, Bogota, causing no injuries but damaging a highway overpass and other structures, police said. The government has been in preliminary talks with the group to hold fully fledged peace negotiations with them as well as the FARC. (Additional reporting by Monica Garcia; Editing by Paul Tait)