ISLAMABAD: Diplomatic relations between Pakistan and Iran hit a new low Tuesday as Islamabad voiced “serious concern” over a warning by Tehran that it may send troops across the border to secure the release of kidnapped border guards.

The five Iranian guards were kidnapped on February 6 in Iran's restive Sistan-Baluchestan province by militants who allegedly took them across the border to Pakistan.

Iran has denounced what it called Pakistan's inability to secure its own borders, with Interior Minister Abdolreza Rahmani-Fazli reportedly warning that Tehran may send forces into Pakistan to free the border guards.

“Pakistan regrets the suggestions of negligence on its part over the incident, especially when Pakistan's active support against terrorist groups in the past, is well-known and acknowledged by Iran,” a Pakistani government statement said.

“Pakistan has already informed the Iranian authorities that its Frontier Corps teams have intensively combed the entire region but could not verify the entry or presence of these Iranian border guards on its territory. It is therefore possible that the miscreants along with the abducted border guards are still hiding within the Iranian territory.”

Expressing “serious concern” over the interior minister's reported comments, Islamabad warned that “Iranian forces have no authority to cross our borders in violation of the international law”.

Home to a large Sunni minority and ethnic Baluch in a predominantly Shia country, Sistan-Baluchestan province has been the scene of unrest in recent years.

An insurgent group calling itself Jaish-ul Adl, or Army of Justice, has claimed responsibility for the kidnapping. The group posted pictures on its Facebook page it said were of the soldiers, handcuffed and being held in an unknown location.