Philippine communist rebels waging one of Asia's longest insurgencies said Tuesday a peace pact was likely not achievable before 2019, in a blow to the government's hopes of securing a deal this year.

Ahead of fresh peace talks starting in Italy on Thursday, the National Democratic Front of the Philippines also warned that forces on the ground were urging an end to a ceasefire as they became frustrated with the government's "broken promises".

"The NDFP goes into the third round of formal talks in Rome determined as always to persevere with the peace talks but increasingly troubled by the other party's sincerity," the communists' chief negotiator, Fidel Agcaoili, said in a statement.

The communists have been waging their "national democratic revolution" since 1968 to overthrow a capitalist system that has created one of Asia's biggest rich-poor divides.

At least 30,000 people have died in the conflict, according to the military.

President Rodrigo Duterte, who describes himself as a socialist, has made ending the rebellion one of his top priorities.

He launched the peace process soon after he took office in June last year and installed three communists in his cabinet.

Both sides agreed to a temporary ceasefire during the first round of talks in Norway last August.

The government said then it was aiming for a final peace deal within 12 months.

But Agcaoili said the communists would need at least two more years after negotiators agreed on a series of economic and political reforms before even beginning "serious discussions" on a final peace pact.

Those economic and political reforms have yet to be agreed, and are meant to be discussed in Rome.

Meanwhile, familiar grievances that derailed peace efforts under previous Philippine presidents appear to have returned to haunt the current negotiations.

- ' A poker game' -

The communists have long demanded that hundreds of jailed communists be released before they will consider making major concessions in peace talks.

Duterte released 18 top leaders to kickstart the peace process but the communists are demanding another 434 rebels be freed immediately.

Duterte said last month he did not want to release the rebels straight away, describing the negotiations as a "poker game".

"My aces are in prison," he said, referring to the jailed communists.

"If I released them all my cards would lost. There would be nothing else to talk about."

The government's chief negotiator, Silvestre Bello, gave a more upbeat assessment of the peace process when contacted by AFP on Tuesday for reaction to Agcaoili's statement.

He said the government was still sticking to its timeframe to secure a peace deal of between nine and 12 months from the start of the talks in August last year.

Bello said he had told the communists it was to their advantage not to delay, or they would risk not being able to finalise a peace pact before Duterte's six-year term ends in 2022.

Agcaoili said the prospect of a permanent ceasefire was "growing dim" because of the prisoner issue. He also accused security forces of violating the current temporary ceasefire.

But Bello said he would still push at the Rome talks, which will last until January 25, for agreement on a permanent ceasefire.

"The prisoners issue has no bearing on the signing of a bilateral ceasefire," Bello said.

The government estimates the rebels' armed wing the New People's Army has about 4,000 fighters, down from a peak of 26,000 in the 1980s.

But they remain particularly active in rural areas of the archipelago, where they are well known for extorting money from local businesses. Their regular deadly attacks on police and military forces also occasionally reach into urban areas.