The Revolutionary Guards were arrested in the Mashkhel area on the border with Iran eight days after a suicide bomber killed 42 people, including six Revolutionary Guard commanders, in Iran's southeastern Sistan-Baluchestan province.
A Sunni Muslim group Jundollah (God's soldiers), claimed responsibility for the blast.
Iran says the group operates from across the border in Pakistan. On Tuesday last week, a senior Revolutionary Guards commander said his force should be given permission to confront terrorists inside Pakistan, state media reported.
"It's a serious matter. We are investigating why they crossed into our territory," said a Pakistani border security official, who declined to be identified as he is not authorized to speak to the media.
Another Pakistani security official said Iranian border officials had told them that the encroachment was accidental and happened after the Guards launched an operation against Jundollah militants near the border.
Iranian state television, however, quoted police as saying eight soldiers and police officers were arrested by Pakistan while they were chasing people suspected of smuggling fuel.
"Five soldiers, three non-commissioned police officers who were chasing fuel smugglers at Iran's border with Pakistan have been arrested in Pakistan. Also two of the smugglers were arrested by Pakistani guards," said a police official, identified only as Brigadier Rezai.
Relations between Iran and Pakistan have been generally good in recent years and the neighbors are cooperating on plans to build a natural gas pipeline but Iran has said last week's suicide bombing would affect relations.
Iran says the Jundollah group has bases in Pakistan and it has urged Pakistan to hand over its leader, Abdolmalik Rigi.
Pakistan has assured Iran that it would cooperate in tracking down and punishing those behind the attack. It has denied that Rigi was in Pakistan.
Iran also accuses the United States and Britain of backing Jundollah.
"Those who commit these bloody terrorist activities, directly or indirectly are related to foreign countries," state television quoted Supreme leader Ayatollah Ali Khamenei as saying.
He was speaking a day after the two suicide bombs killed more than 150 people in Baghdad, in one of Iraq's bloodiest attacks in years.
(Additional reporting by TEHRAN bureau; Editing by Sanjeev Miglani)
