SAN SALVADOR (Reuters) - Jose Miguel Insulza, the secretary general of the Organization of American States, gave his support to a fragile truce between El Salvador's gangs on Thursday, despite a recent uptick in murders.

After meeting with El Salvador's president, Mauricio Funes, Insulza backed the tentative peace treaty between the Mara Salvatrucha, or MS-13, and their rivals, Barrio 18, pointing to an overall decrease in the number of killings in the Central American country since the truce took hold in March 2012.

"It's a process that will always have highs and lows, but for our organization, the most important thing is the substantial reduction we've seen in El Salvador's murder rate," Insulza told reporters.

The truce, which is backed by the Catholic Church, aims to reduce the number of homicides. According to the United Nations, El Salvador is the world's most-violent nation, with a homicide rate of 66 per 100,000 inhabitants in 2011.

The unprecedented ceasefire helped bring murders down to an average of five per day from 12 before the agreement.

But killings have been rising since late May, with homicides averaging 16 per day in early July, with one week that month notching 103 murders.

(Reporting by Nelson Renteria; Writing by Gabriel Stargardter; Editing by Eric Beech)