Colombia's President Juan Manuel Santos remains frontrunner to win a second term in office in next month's election, but his two closest rivals are gaining ground, according to a poll published on Sunday.

Twenty-three percent said they would vote for the center-right incumbent in the first round of voting on May 25, one percentage point less than in the last poll in March and five less than two months ago, according to the survey by Ipsos Napoleon Franco published in weekly Semana magazine.

A lack of clear message in the campaign combined with lingering anger over his handling of a national protest last year in the farm sector contributed to a decline in support for Santos, the poll said. A perceived slowdown in peace talks with Marxist FARC rebels to bring an end to five decades of conflict has also raised concern among voters.

Support for Santos, an economist and former journalist, has started to slow as his rivals pick up speed. If no candidate wins more than 50 percent, the two leading contenders will go to a runoff on June 15.

Backing for right-wing contender Oscar Ivan Zuluaga, who represents former President Alvaro Uribe's party, has risen. He would garner 15 percent of the vote in the first round next month, up from the 9 percent he polled in March and 8 percent in February. The advantage Santos has over Zuluaga has closed to 8 points from 20 in February.

A strong result in March congressional elections for Uribe, who became a senator, and his Democratic Center party overall, helped raise Zuluaga's national profile and boost his support, the poll said.

Zuluaga and Uribe fiercely oppose peace talks with the FARC, or Revolutionary Armed Forces of Colombia, after the former leader spent two terms inflicting heavy military blows on the rebels with U.S. backing.

Zuluaga's campaign is wooing those voters who would prefer the government to defeat them militarily.

Backing for former Bogota Mayor Enrique Penalosa has risen to 11 percent from 8 percent in March and doubled since the February poll, the survey said. Penalosa is the candidate of the Green Alliance, not an ecology-focused party despite its name and one with several prominent left-wing members.

Still, Santos would beat both his leading rivals in a second round of voting, the poll showed.

In a runoff between Santos and Penalosa, the former mayor would clinch 24 percent of the ballot against 33 percent for Santos, while Zuluaga would win 23 percent against 34 percent for the president.

Santos took the controversial step in his first term of engaging in peace talks with the Andean nation's main left-wing guerrilla group, the FARC, in late 2012, the poll showed.

Concern over the talks has risen. In April, 63 percent were pessimistic there will be positive outcome, against 58 percent in March and 57 percent in February, the poll said.

The Ipsos poll interviewed 1,208 people between April 21 and 23 and had a margin of error of 2.8 percent. The survey was conducted using a mixture of face-to-face and telephone interviews.