US President Barack Obama recently ordered the National Security Agency to curtail eavesdropping on the United Nations headquarters in New York as part of a review of US electronic surveillance, according to a US official familiar with the decision.

Obama's order is the latest known move by the White House to limit the NSA's vast intelligence collection, in the wake of protests by allies, including German Chancellor Angela Merkel, over US spying on foreign heads of state.

The full extent of US eavesdropping on the United Nations is not publicly known, nor is it clear whether the United States has stopped all monitoring of diplomats assigned to the UN in New York or elsewhere around the world.

"The United States is not conducting electronic surveillance targeting the United Nations headquarters in New York," said a senior Obama administration official, who spoke on condition of anonymity.

The official did not address past surveillance of the world body. Such programmes are highly classified, although some details have been leaked by former NSA contractor Edward Snowden.

According to the first official, the president's aides have said in briefings that the White House no longer wanted to conduct certain monitoring of UN targets. The official said that the decision was made within the last few weeks.

The NSA declined to comment on the matter. Spokesmen for UN Secretary General Ban Ki-moon did not respond immediately to a request for comment.

Historically, the United Nations and New York-based diplomatic missions of member states have been targets for aggressive spying by the United States, its allies and adversaries. FBI counter-intelligence squads have long mounted operations to try to identify spies posing as UN diplomats.

The German news magazine Der Spiegel, citing documents leaked by Snowden, reported in August that the NSA had succeeded in the summer of 2012 in getting into the UN video conferencing system and breaking its encryption.

"The data traffic gives us internal video teleconferences of the United Nations," Der Spiegel quoted one NSA document as saying. It added that within three weeks the number of decoded communications had risen to 458 from 12.

Reuters was unable to independently confirm the document.

The White House has said it is undertaking a broad review of US intelligence collection programmes to determine whether they are appropriate.

"The Administration's review is ongoing so I'm not in a position to discuss the details or the outcomes, but we have already made some decisions through this process and expect to make more as we continue," National Security Council spokeswoman Caitlin Hayden said in a statement late on Monday.

The statement was issued after Senator Dianne Feinstein, chairman of the Senate Intelligence Committee, complained about the reported surveillance of Merkel and other US allies, and announced her own deep review of US intelligence collection programmes.

Feinstein said that she understood Obama had not known that Merkel's communications were being collected since 2002. "That is a big problem," she said.

"The White House has informed me that collection on our allies will not continue, which I support," Feinstein said.

Der Spiegel set off the latest furor over US eavesdropping last week when it approached the German government for comment on an entry in an NSA targeting document, prepared last spring and supplied by Snowden, which indicated that a phone number later confirmed to be that of Merkel's cellphone was on the target list. Under the number was a notation which listed the number's subscriber as "GE Chancellor Merkel."

Current and former officials familiar with NSA practices said that the agency for years has eavesdropped on foreign leaders, friendly and unfriendly, and that such activities have been repeatedly briefed to congressional intelligence committees, at least in outline.

The officials said it was possible, however, that neither intelligence committee leaders like Feinstein, nor Obama, knew specifically which foreign officials might have been targeted at any particular time. - Reuters