CARACAS, Venezuela (AP) — Venezuela's oil minister said Friday that security forces have thwarted an attack on a key refinery as the nation with vast oil reserves struggles to meet its domestic gasoline demands.

Two Venezuelans were arrested Thursday and investigators confiscated explosives the suspects planned to use in an attack on El Palito refinery on the nation's northern Caribbean coast, Minister of Petroleum Tareck El Aissami said in a televised announcement.

Officials showed images of explosives and other material as proof of what El Aissami called a terrorist plot. He said the two suspects confessed to taking orders from Colombian officials.

Aissami called upon Venezuelan oil workers to be vigilant amid an ongoing threat from international adversaries such as the United States and Colombia to harm the South American nation.

Once-wealthy Venezuela holds the world’s largest oil reserves, but the state-run oil industry is in collapse — something critics blame on mismanagement and corruption and the government attributes to sabotage and U.S. sanctions aimed at driving socialist President Nicolás Maduro from power.

Venezuela recently has been relying on fuel shipments from Iran, another U.S. adversary.