Pakistani forces detained 11 Iranian Revolutionary Guards on Monday for crossing into Pakistan days after an Iranian commander was reported saying his men should be allowed to confront terrorists in Pakistan.
The 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.
The Iranians were being detained and it was not clear when they would be released, the Pakistani officials said.
Iran says the Jundollah group has bases in Pakistan and it has urged Pakistan to hand over its leader, Abdolmalik Rigi.
Pakistan condemned the October 18 bombing and denied suggestions from the Iranian president that "some security agents" in Pakistan were cooperating with the bombers.
Pakistan also denied that Rigi was in Pakistan.
Impact on relations?
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.
Pakistan has assured Iran that it would cooperate in tracking down and punishing those behind the attack.
Iran also accuses the United States and Britain of backing Jundollah.
Stephane Dudoignon, a Western expert on Sistan-Baluchestan, told Reuters in an interview last week the rise of Jundollah coincided with an explosion in drug smuggling from which it earned much of its funding.
Jundollah, Dudoignon said, drew its religious ideology from Deobandi Islam, a traditionalist Sunni school of thought which emerged in British India in the 19th century and has since spread across Pakistan and Afghanistan.
Analysts say its use of suicide bombings suggest it is increasingly influenced by the sectarian anti-Shi'ite agenda of some militant groups in Pakistan which also follow a Deobandi tradition -- as do the Afghan Taliban.