Five men suspected of involvement in the attack on Army Public School, have been arrested across the border in Afghanistan, the BBC reported.
The arrests came after Pakistan supplied information to the Afghan government.
Afghan officials denied reports that the five had been handed over to Pakistan, saying they would be questioned in Afghanistan first.
An Afghan security official said the five had been arrested in an eastern province, BBC said.
The arrests confirm there has been a recent thaw in relations between the Afghan and Pakistani governments, BBC said quoting its correspondent in Kabul.
Afghan President Ashraf Ghani made a successful trip to Pakistan soon after coming into office last October and both sides have promised better cross-border cooperation, it said.
Afghan security officials believe the five arrested men were indirectly involved in the school massacre on December 16. All seven gunmen attacking the APS were killed.