Colombia resumed its path toward peace with leftist rebels Monday, bolstered by President Juan Manuel Santos' victory in elections seen as a referendum on his bid to end the 50-year-old conflict.

Santos picked up 50.95 percent in Sunday's runoff against the more conservative Oscar Ivan Zuluaga, validating his insistence on continued negotiations with the Revolutionary Armed Forces of Colombia and the smaller National Liberation Army.

Santos, a center-right politician who received the backing of some leftist parties, had staked his presidency on a negotiated peace with the FARC.

The president's approach was challenged by Zuluaga, who called for stricter conditions as a prerequisite for any accord with the rebels.

Founded in the 1960s, the ELN and the FARC have been perennial protagonists in Latin America's longest guerrilla war. They boast 2,500 and 8,000 fighters respectively.

The conflict, with its violent cocktail of rebels, paramilitary militia and criminal gangs, has left more than 220,000 people dead and forced five million to leave their homes over the past 50 years.

In his victory speech, Santos promised a "fair peace" without "impunity."

But he admitted that "we will have difficult moments to guarantee that it is not only fair but lasting."

"The peace process emerges stronger and more and more viable," said Jorge Alberto Restrepo, head of CERAC, the Conflict Analysis Resource Center. "Even the FARC and the ELN appear strengthened as negotiators."

Norway, who has acted as a facilitator for the talks, which have been underway in Cuba since November 2012, hailed the Santos victory as a historic opportunity to reach peace.

Santos now has a good platform for the continuation of the negotiations, said Foreign Minister Boerge Brende.

UN chief Ban Ki-moon echoed that the opportunity "should not be missed."

"Today, Colombia is closer than ever to ending the hemisphere's longest armed conflict through negotiations," he said.

But the president-elect still faces formidable challenges.

"Santos got a boost from many voters who cast ballots for him because they want the peace process to continue, not necessarily because they support what his government has done" more generally, said University of the Andes political analyst Felipe Botero.

Though the economy is growing at a pace of more than four percent a year, a third of Colombia's 47 million residents live in poverty.

- Role for the opposition? -

Santos, whose swearing-in takes place in August, will now have to define a strategy going forward that takes into account his election alliances with the left.

"The government finds itself with political 'I owe yous' vis a vis certain political sectors and the rebels themselves who helped him during his campaign," said historian Ruben Dario Acevedo.

It also remains to be seen what role will be played by the right-wing opposition, embodied by Zuluaga who garnered 45.02 percent of the votes cast. The remainder of the ballots were blank protest votes.

During the campaign, Zuluaga tapped into a strong current of skepticism about the FARC's intentions.

His showing, including a first round victory over Santos, owed much to his mentor, president Alvaro Uribe, a popular former president who weakened the FARC militarily during his 2002 to 2010 presidency.

Charging "fraud" in Sunday's elections, Uribe declared that his hardline Democratic Center party would stay "loyal to its principles."

"Proud to have been the candidate of Uribism," the 55-year-old Zuluaga said.

According to Botero, "Uribism is becoming an important force that will oppose Santos in parliament, watching the peace talks very closely."

Marta Lucia Ramirez, the conservative party presidential candidate who threw her support to Uribe's camp after the first round of voting in May, also signaled her intention to "continue to insist that all negotiations take place amid conditions."