MANILA - President Rodrigo Duterte phoned a top communist leader promising to free 60 political prisoners by the end of the year, a source involved in the negotiations told ABS-CBN News Tuesday.

Half of the them will be released before Christmas, Duterte told the Netherlands-based Jose Maria Sison, founder of the Communist Party of the Philippines, the source said.

When to release other political detainees will be tackled when formal peace talks resume in Rome from Jan. 18 to 25, the source said.

Another source assisting in the negotiations also confirmed the phone conversation late Monday between Duterte and Sison.

• READ: Peace talks with Reds still on despite 'hiccup' in rebels' release

Sought for comment, Edre Olalia, a legal consultant for the rebels, said the government should "go beyond press releases" and just free the detainees.

"Just do it," he told ABS-CBN News.

The National Democratic Front of the Philippines is seeking freedom for some 400 political prisoners, 130 of whom are said to be sick and aging.

Duterte reached out to Sison, NDFP's chief political consultant, to help ensure no armed encounter would take place between government troops and communist rebels during the holidays, the source said.

The phone call, which lasted for less than 10 minutes, was also meant to move the peace talks forward as rebels refused to sign a bilateral ceasefire agreement until all political detainees were freed, the source said.

Duterte earlier promised to release political prisoners within 48 hours if presented a signed document binding both sides to a common ceasefire.