Attackers blow up trucks at Colombia's biggest coal exporter

* No casualties in attack but four trucks damaged

* Company and striking union condemn assault at mine

* Union says pre-talks on strike still on for Monday

By Jack Kimball

BOGOTA, Feb 24 (Reuters) - Unknown assailants blew up four trucks belonging to Colombia's largest coal exporter, Cerrejon, on Sunday in the latest attack against the mining sector in the world's fourth-largest coal exporter.

Cerrejon - a joint venture between Anglo American, BHP Billiton and Xstrata - said it had suffered "a terrorist attack" at its Mina Sur mining area in the northern province of Guajira.

"There were no casualties, but four company trucks were seriously affected, which were idle as a result of the strike affecting the company for 17 days," it said in a statement.

The assault comes a day before Cerrejon and workers, who have been on strike since Feb. 7, plan to restart stalled wage negotiations to try to end a walkout that costs Colombia $1 million in royalties and a 100,000 tonnes in lost output daily.

The company did not speculate on who might have launched the attack, which the Sintracarbon labor union was also quick to condemn.

"Our union and I personally raise our voice to strongly reject these deeds, which are also an attack against the strike," union leader Igor Diaz wrote in a message on Twitter.

He said that talks on Monday to finalize logistical issues and then re-start substantive wage negotiations would go ahead in spite of the attack.

"The process continues tomorrow, we reject and oppose these acts and no one will stop the search for an agreement," he said.

Oil and mining operations have been targets for armed rebels as well as labor unrest, although security for business has vastly improved since a U.S.-backed offensive against anti-government guerrillas was launched in 2002.

Improved security has helped attract billions of dollars in new investment as explorers push into more areas in search of minerals and oil. In 2012, Colombia brought in $16 billion in foreign investment, up from around $2 billion in 2002.

Cerrejon's problems have added to the woes of the Colombian coal sector, which have included a ban on trains running at night due to noise pollution in the main coal-producing province of Cesar and the suspension of coal loading at Drummond, the other top Colombian exporter.

Analysts said the apparent global abundance of coal and weak demand in key markets may mean buyers have been cushioned from big price rises despite multiple disruptions to shipments.