President Hugo Chavez on Sunday promised Venezuela would respond militarily if Colombia violates its border, where he ordered tanks as well as thousands of troops. He also ordered closed Venezuela's embassy in Bogota.
"This could be the start of a war in South America," Chavez said.
Ecuadorean President Rafael Correa said he deployed troops to the border while also withdrawing his government's ambassador from Bogota and expelling Colombia's top diplomat.
"There is no justification," Correa said Sunday night, snubbing an earlier announcement from Colombia that it would apologize for the military incursion.
Chavez called the killing of rebel leader and spokesman Raul Reyes and 16 other guerrillas on Saturday an attack by a "terrorist state," saying it shows Colombian President Alvaro Uribe is a "criminal."
"Mr. Defense Minister, move 10 battalions to the border with Colombia for me, immediately - tank battalions. Deploy the air force," Chavez said during his weekly TV and radio program. "We don't want war, but we aren't going to permit the U.S. empire, which is the master (of Colombia) ... to come divide us."
Correa said Colombia deliberately carried out the strike beyond its borders, flying deep into Ecuador to bomb the rebel camp from the south. The Ecuadorean leader said the rebels were "bombed and massacred as they slept, using precision technology."
The Colombian military said the camp was located just over a mile from the border.
Colombian officials have long complained that rebels are allowed to take refuge across its borders in both Ecuador and Venezuela.
Colombia said after the assault that FARC "terrorists" including Reyes "have had the custom of killing in Colombia and taking refuge in the territory of neighboring countries."
Fidel Castro blamed the U.S. for the turmoil, writing on Monday that "genocidal plans of the Yankee empire" created tensions between the South American nations.
"We can plainly hear the trumpets of war to the south of our continent as a consequence of genocidal plans of the Yankee empire," Castro wrote in an essay published in the Communist Party daily Granma. The older Castro is a socialist ally of Chavez, and the oil-rich South American nation has helped keep Cuba's economy growing by providing nearly 100,000 barrels of subsidized oil a day to Cuba.
In Chile, President Michelle Bachelet offered to mediate in the conflict.
"A situation like this requires an explanation from Colombia to Ecuadoreans, to the Ecuadorean president and to the entire region," Bachelet said. "We are very worried."
Mexico's President Felipe Calderon also offered to helps and urged the countries to begin talking "within the framework of the appropriate regional organizations."
Ecuadorean troops recovered the seminude bodies of 15 rebels in their jungle camp.
Soldiers covered their faces with bandannas to ward off the stench Sunday at the camp, where bodies were splayed on the ground in their underwear. Scattered among the corpses were pieces of clothing, shoes, guns, grenades and a refrigerator.
Soldiers also found three wounded women at the camp - a Mexican philosophy student injured by shrapnel and two Colombians - who were evacuated by helicopter to be treated. Ecuadorean officials said they believe there were two bomb attacks on the camp.
Colombian commandos removed the cadavers of Reyes and one other rebel.
"This could be the start of a war in South America," Chavez said. He warned Uribe: "If it occurs to you to do this in Venezuela, President Uribe, I'll send some Sukhois" - Russian warplanes recently bought by Venezuela.
"This is saber-rattling, trying to make a point," said Adam Isacson, an analyst for the Washington-based Center for International Policy. By holding a moment of silence in honor of the slain rebels during his program, Chavez "has all but said that the FARC will be safe in Venezuela, and that the Venezuelan armed forces would respond to a similar Colombian incursion into Venezuelan territory."
However, Isacson said, the countries share robust trade, the militaries "are not enthusiastic" and the populations of the neighbors "are hardly consumed by war fever."
The situation pushed tense relations between Venezuela and Colombia to a new nadir, though cross-border trade has not yet been seriously affected.
Colombian Vice President Francisco Santos told the U.N. Human Rights Council in Geneva that his government will not accept any provocation that could put the region's stability at risk. In a speech to the body, Santos said Colombia will stand firm in its fight against terrorism. He did not mention Ecuador or Venezuela by name.
Colombia's police commander, Gen. Oscar Naranjo, said documents from a computer seized where Reyes was killed suggested Ecuador's president is deepening relations with the FARC.
The two documents, copies of which were obtained independently by The Associated Press, were apparently written by Reyes in the past two months and addressed to the FARC high command. An Ecuadorean government spokesman called the Colombian claims a lie.
There were no concrete reports on troop movements in Venezuela's state media early Monday. Chavez did not specify how many troops he was sending to the border. A Venezuelan battalion traditionally has roughly 600 soldiers.
Chavez has increasingly revealed his sympathies for the FARC, and in January asked that it be struck from lists of terrorist groups internationally.
The leftist FARC has been fighting Colombia's government for more than four decades, and funds itself largely through the cocaine trade and kidnaps for ransom and political ends.
How exactly Reyes was killed was not immediately clear.
Colombian Defense Minister Juan Manuel Santos said military commandos, tracking Reyes through an informant, first bombed a camp on the Colombian side of the Ecuadorean border. He said the troops came under fire from across the border in Ecuador and encountered Reyes' body when they overran that camp.
"It was a massacre," said Correa, who accused Colombia of lying and said some rebels were shot in the back.
Colombia and Venezuela have been locked in a diplomatic crisis since Uribe sought in November to halt Chavez's efforts to mediate a prisoner swap. The FARC has since freed six hostages to delegates of Chavez, including four released last week.
The FARC has demanded that a safe zone be created in Colombia to negotiate a swap of some 40 high-value captives, including former presidential candidate Ingrid Betancourt and three U.S. defense contractors, for hundreds of imprisoned guerrillas.