In a sudden blow to peace talks with FARC rebels, Colombian President Juan Manuel Santos has broken off negotiations with the guerrillas after the kidnapping of an army general and two companions along a jungle river.

Brig. Gen. Rubén Alzate was traveling with a soldier and a government attorney when the three were allegedly captured Sunday by members of the Revolutionary Armed Forces of Colombia outside Quibdo, capital of the northwestern province of Choco.

Santos said he did not know what Alzate and his companions were doing in the area without a military escort, nor why Alzate was dressed as a civilian and apparently disregarded warnings by the captain of the boat he was traveling in not to venture too far downriver into rebel territory.

But in a televised speech Monday night he warned the FARC that the abduction had left the peace effort in peril. “This is the moment for them to demonstrate their commitment to the process,” he said, demanding the captives’ immediate release.

FARC commanders, many of whom are in Cuba for the peace talks, have not said whether they are holding Alzate, but he would be the highest-ranking military officer captured by the rebels in their half-century guerrilla war against the government.

This handout picture taken Aug. 15 shows Colombian Army General Rubén Alzate, who kidnapped Nov. 16, Colombian President Juan Manuel Santos said. (Colombian Army Press Office via AFP/Getty Images)

Colombian negotiators were due to resume talks with the FARC leaders in Havana this week, but Santos said he has ordered them to cancel the trip. The peace process will not resume until the three are released, he said, ordering Colombia’s defense forces to mount a rescue operation.

The two-year-old negotiations have advanced further than any previous attempt to end the civil conflict, but fighting between the government and FARC guerrillas has continued, and the rebels captured two soldiers last week in another part of the country.

Having staked his presidency on the outcome of the armistice attempt, Santos has been facing a decline in his approval rating and a barrage of criticism from opponents, led by former president and FARC archenemy Álvaro Uribe.

“There is no way that Colombian public opinion, Colombia’s political class, or especially Colombia’s military will allow talks to go on while FARC is holding a general,” said Adam Isacson, a Colombia expert at the Washington Office on Latin America, describing the incident as a potential “deal-breaker.”

Isacson noted that the commander of the FARC’s 34th Front, the group allegedly holding Alzate, is in Havana with many of the top insurgency leaders, which could make it easier to apply political pressure for a swift release.