About once a month the Central Intelligence Agency sends a fax to a general at Pakistan's intelligence service outlining broad areas where the US intends to conduct strikes with drone aircraft, according to US officials.

The Pakistanis, who in public oppose the program, don't respond.

This was stated in a report published by the Wall Street Journal.

The fax would not mention any specific target but outline the boundaries of the airspace the drones would use—large areas along the Pakistan-Afghanistan border referred to as flight "boxes" because they are shaped like three-dimensional rectangles in the sky, the report stated.

On this basis, plus the fact that Pakistan continues to clear airspace in the targeted areas, the US government assumes it has tacit consent to conduct strikes within the borders of a sovereign nation, according to officials familiar with the program.

In public speeches, Obama administration officials have portrayed the US' use of drones to kill wanted militants around the world as being on firm legal ground. In those speeches, officials stopped short of directly discussing the CIA's drone program in Pakistan because the operations are covert.

According to the report, the US justifies the legality of its drone attacks by citing Pakistan’s silence as tacit consent  but it is  also concerned about setting precedents for other countries, including Russia or China, that might conduct targeted killings as such weapons proliferate in the future.

Because there is little precedent for the classified US drone program, international law doesn't speak directly to how it might operate. That makes the question of securing consent all the more critical, legal specialists say.

Pakistan also has considered challenging the legality of the program at the United Nations.

"No country and no people have suffered more in the epic struggle against terrorism than Pakistan," Pakistan President Asif Ali Zardari told the UN General Assembly Tuesday. "Drone strikes and civilian casualties on our territory add to the complexity of our battle for hearts and minds through this epic struggle."

The report said that Pakistan believes the CIA continues to send notifications for the sole purpose of giving it legal cover.

Some in the US also worry about the possibility of Pakistan playing both sides as  a lack of a Pakistani response to US notifications might be a way for Pakistan to meet seemingly contradictory goals i.e  on one hand it lets the CIA continue using its airspace but on the other hand also distancing the government of Pakistan from the program, which is deeply unpopular among Pakistanis.

Government consent provides the firmest legal footing, legal experts say. The US has that in Yemen, whose government assists with US strikes against an al Qaeda affiliate. In Somalia, the nominal government, which controls little territory, has welcomed US military strikes against militants.

In an April speech, White House counterterrorism adviser John Brennan said the administration has concluded there is nothing in international law barring the US from using lethal force against a threat to the US, despite the absence of a declared war, provided the country involved consents or is unable or unwilling to take action against the threat.

John Bellinger, the top State Department legal adviser in the George W. Bush administration, was of the view that the US is "not unreasonable to assume consent" from Pakistan for the use of drones, "particularly when the US conducts repeated attacks and it's open and obvious."

Giving details of the procedure the reports says that there was a more open channel of communication, until the raid that killed Osama bin Laden prior to which the ISI would send back a fax acknowledging receipt.

The return messages stopped short of endorsing drone strikes. But in US eyes the fax response combined with the continued clearing of airspace to avoid midair collisions—a process known as "de-confliction"—represented Pakistan's tacit consent to the program but after the OBL raid the ISI stopped acknowledgement receipt of the drone notifications.

According to the report, US officials believe that the ISI  chose that option knowing an outright denial of drone permission would spark a confrontation, and also believing that withdrawing consent wouldn't end the strikes.

Ambiguity over the drone attacks is two-sided as  the US agency general counsels have drawn the line at revealing detailed criteria for picking targets or disclosing who makes the decisions. Leaving these things ambiguous could help shield officials involved against possible court challenges and avoid providing information that militants could use to evade targeting.