Pakistan and Bangladesh on May 12 summoned each other's ambassadors to register "strong protest" in a dispute over the execution of an Islamist leader in Bangladesh.
The two Muslim nations used to be two halves of the same country until Bangladesh broke away in a 1971 war of independence.
Bangladesh has in the past few years been prosecuting people accused of carrying out crimes in support of Pakistani forces during the war, and has executed five of them -- the most recent one, Motiur Rehman Nizami, on May 11.
Pakistan called Nizami's hanging "unfortunate" and said attempts by Bangladesh to malign Pakistan were "regrettable."
Bangladesh retorted that it "deeply regrets" Pakistan's "malicious campaign...against the trials of the crimes against humanity and genocide in Bangladesh."
Relations between the two countries have never recovered from the 1971 war in which Bangladesh claims that 3 million people were killed.
The war crimes tribunal that sentenced Nizami, a member of the Jamaat-e-Islami Islamist party, has been criticized as not up to international standards by human rights groups, but the trials are supported by many in Bangladesh.