Amid an unexpected fight over US surveillance powers from congressional Republicans, the National Security Agency has agreed to curb its highly controversial collection of Americans’ emails that discuss foreign intelligence targets, although how comprehensive that stoppage is remains unclear.

According to a US official directly familiar with the decision, the NSA has agreed to cease so-called “about” surveillance under a critical 2008 legal authority, known as section 702 of the Foreign Intelligence Surveillance Act (Fisa).

Yet the NSA has other authorities available to it for collecting substantial amounts of the same sort of American communications, including a Ronald Reagan-era executive order, known as 12333. The NSA has not indicated whether or not the “about” collection will cease wholesale, or merely migrate to a different legal authority.

Though the NSA continues to defend the legality of the surveillance it is curbing, it conceded on Friday that the decision follows an internal review that determined it violated constraints agreed to with a secret surveillance court. It called those violations “inadvertent”.

The US surveillance juggernaut has assured intelligence officials that it will limit its vast interception of US communications that transit the internet, known as “upstream” collection, under section 702 to those messages sent from or received by foreign intelligence targets.

It portrayed the decision, first reported by the New York Times, as an optional measure to protect Americans’ privacy, while not conceding a key point of its critics: that such “about” collection violated Americans’ constitutional rights to privacy.

Though the NSA continues to insist “about” collection is legal, the intelligence agency has pledged to delete the “vast majority of its upstream internet data”, it said in a statement.

“The changes in policy followed an in-house review of Section 702 activities in which NSA discovered several inadvertent compliance lapses”, the agency said, which it reported to Congress and the foreign intelligence surveillance court.

It is far from the first time the NSA has conceded that its vast surveillance powers under section 702, a surveillance authority expiring in December, have surpassed the boundaries set with the Fisa court.

A Fisa court decision from 2011, declassified in 2013, found that the agency had overcollected tens of thousands of purely domestic US emails in violation of the law, which permits warrantless interception of Americans’ international communications so long as one party to the communication is a foreigner overseas.

The NSA at the time represented the 702 overcollection as an unavoidable consequence of its collection technology – a limit it cited on Friday to warn that the agency could not fully purge its hoards of data that it now pledges no longer to collect.

“Because of the limits of its current technology, [NSA] is unable to completely eliminate ‘about’ communications from its upstream 702 collection without also excluding some of the relevant communications directly ‘to or from’ its foreign intelligence targets. That limitation remains even today,” it said.

Yet the NSA’s authorities under executive order 12333 are vast, undisclosed and unconstrained by any need to explain its collections to the Fisa court. A former state department official who has warned Congress about 12333, John Napier Tye, has alleged that the NSA uses 12333 as a backup plan to route around legal restrictions on US surveillance.

“To the extent US person information is either stored outside the United States, routed outside the United States, in transit outside the United States, it’s possible for it to be incidentally collected under 12333,” Tye told the Guardian in 2014.

It is unclear to sources briefed on the matter whether such surveillance routing to the executive order is in effect.

But the decision to limit collection under 702 comes amid an unexpected political backdrop: resistance to renewing the expiring statute by the congressional Republicans charged with championing it on Capitol Hill.

With the Trump administration incensed at leaks over its communications with Russian officials that it blames on US intelligence, Republicans on the House intelligence committee have openly warned NSA that they cannot guarantee the votes for renewing the controversial surveillance power without a leak crackdown.

Since 2008, the Republicans have typically led a defense of 702 powers, particularly against the revelations of widespread surveillance provided by Edward Snowden. But in a partisan reversal, intelligence-panel Democrats on Friday looked past the admitted NSA violations and called for the statute’s renewal.

“Going forward, I will continue to expect strict compliance with the Fisa court orders and will push for Section 702’s reauthorization along with any additional reforms needed to further strengthen and institutionalize protections for privacy and transparency,” said Adam Schiff of California, the senior Democrat on the House intelligence committee.

Schiff’s Senate counterpart, Virginia Democrat Mark Warner, added: “I believe we can now look forward to Congress and, in particular, the Senate intelligence committee on which I serve as vice-chairman, quickly turning to the consideration and debate of this critical authority prior to its expiration set for December 31, 2017.”

Civil libertarians hailed the NSA decision while warning that surveillance checks need to go further.

“While we welcome the voluntary stopping of this practice, it’s clear that Section 702 must be reformed so that the government cannot collect this information in the future,” said Michelle Richardson of the Center for Democracy and Technology.