U.S. Ambassador Cameron Munter was briefly detained at a Pakistani airport earlier this week, Pakistan's Dawn newspaper reported Sunday.

The incident comes as relations between the two countries are already at a low point following the May 2 raid on Osama Bin Laden's compound, which was conducted without Pakistan's knowledge. Munter was briefly detained in Islamabad on his way to Karachi when officials asked him to produce a document, called a no-objection certificate, that allows diplomats to travel to other parts of the country.

Munter reportedly had the document on him, the newspaper reported, but the embassy protested what it sees as a violation of international law obligating a host state to “ensure to all members of the mission freedom of movement and travel in its territory.”

Pakistani officials said the certificates exist for the security of diplomats. They said they weren’t targeting American diplomats.

But one security official anonymously told the newspaper that the travel restrictions were now being enforced because under-cover foreign intelligence agents have been assigned to Pakistan while posing as diplomats. The enhanced enforcement is clearly aimed at the CIA, the newspaper reported.