One former senior intelligence official said he had been unaware there were any exceptions to the policy of the five nations sharing intelligence information with each other, but said he would be surprised if the United States chose to spy on its closest allies very frequently.

“They would do this unilaterally so rarely and in such extraordinary circumstances because they would be so concerned about hurting the relationship,” said the former official, who spoke only on condition of anonymity. “My bet is that they wouldn’t go to that well very often.”

The memo contains several protocols on who should be alerted, and under what circumstances, when spying must take place on other Five Eyes countries — also referred to as “Second Party” countries.

One paragraph, marked secret, appears to suggest that the preferred option is to gain permission from the country whose citizens are to be spied upon. But the very next paragraph, marked secret and NOFORN, indicates that the N.S.A. can go it alone if permission is not forthcoming — or if United States chooses not to ask.

“When sharing the planned targeting information with a Second Party would be contrary to U.S. interests, or when the Second Party declines a collaboration proposal, the proposed targeting must be presented to the Signals Intelligence Director for approval with justification for the criticality of the proposed collection,” the passage explains.

It goes on to say that if that spying is approved, the information it gleans “must be maintained in NOFORN channels” — i.e., never shared with the spied-upon country.

A footnote goes further, suggesting that if a Five Eyes citizen is outside of his or her own country, the limits are lifted. In that case, the memo says, “there may be no restrictions associated with that collection” outside of basic N.S.A. rules on avoiding spying on innocent Americans and similar guidelines.