The Philippine government and communist rebels said Friday that formal negotiations to end a lengthy insurgency could restart shortly, though the rebels' armed wing announced it was beefing up its guerilla campaign.

Peace talks regarding one of the world's longest-running insurgencies, which have been on-and-off since the 1980s, may resume as early as the second half of January, Communist Party of the Philippines founder Jose Maria Sison said in a video message on Facebook.

Back-channel talks to "prepare the agenda" for formal negotiations have been ongoing since September and agreements on a ceasefire and social and economic reforms may be finished before President Benigno Aquino steps down in 2016, said Sison, who is in exile in the Netherlands.

Teresita Deles, presidential adviser on the peace process, did not give a timeline for the negotiations, but told AFP that Sison's remarks were "very positive" and indicated that common ground between the two sides was "broadening".

"Friends of the peace process have been shuttling between the two parties to explore possible parameters for restarting talks at the earliest time possible," Deles said of the back-channel negotiations.

"So far, feedback has been positive but there remain matters to be clarified in order to ensure that, if ever we do resume talks, it will not go the same way of an early, major impasse that has happened too often in the past," she said.

In a goodwill gesture amid a Christmas ceasefire, the rebels on Friday released two soldiers held captive for four months in the southern province of Bukidnon, military spokesman Brigadier General Restituto Padilla told AFP.

However, the communists' armed wing, the New People's Army (NPA), marked the 46th anniversary of its founding on Friday with a call to intensify its guerilla campaign.

"(We) must seize and control the initiative by launching more frequent and sustained tactical offensives with occasional blows to the head of the enemy," the group said in a statement.

Running for almost half a century, the communist insurgency has claimed 30,000 lives, according to military estimates.

The military declared a month-long ceasefire with the NPA for the Christmas holidays and Pope Francis's scheduled visit in January. The rebels said they would observe a shorter truce.

The NPA's strength has dwindled to 4,000 fighters from a peak of more than 26,000 in the late 1980s, according to the military.

Negotiations under Aquino faltered after the government turned down the rebels' demands that their detained comrades be released.

Separately however, Aquino has succeeded in forging a peace deal with Muslim rebels.

The agreement, signed in March, calls for the expansion of the autonomous Muslim region in the south and the disarming of thousands of guerilla fighters.