(Reuters) - Director of National Intelligence James Clapper said on Monday his office is considering options to obtain and publicly disclose an estimate of the number of U.S. persons caught incidentally in Internet surveillance intended for foreign targets.

“We are looking at several options right now, none of which are optimal,” Clapper told reporters at a breakfast hosted by the Christian Science Monitor news organization. He cautioned the task would be difficult and potentially run afoul of privacy considerations.

The comments came in response to a bipartisan letter sent last week by 14 lawmakers in the U.S. House of Representatives pressing Clapper to provide a public estimate of the number of Americans ensnared in data grabs of foreign Internet communications traffic. They said the estimate is needed to gauge possible reforms to a controversial surveillance law due to expire at the end of 2017.

That law enables an Internet surveillance program known as Prism that was first disclosed in a series of leaks by former National Security Agency contractor Edward Snowden nearly three years ago.

Prism gathers messaging data from Alphabet Inc’s Google , Facebook Inc, Microsoft Corp, Apple Inc and other major tech companies that is sent to and from a foreign target under surveillance.

Intelligence officials say data about Americans is “incidentally” collected during communication with a target reasonably believed to be living overseas. Critics see it as back-door surveillance of Americans without a warrant.

Clapper said the tool “is a prolific producer of critical intelligence” that helps the United States and its allies fend off threats to national security.

He added that rendering an estimate would likely require using “more invasive procedures” that would jeopardize the privacy of Americans implicated in the data grabs.

“If such an estimate were easy to do and explainable without compromise, we would have done it a long time ago,” Clapper said.

In their letter, which echoed earlier calls for an estimate from Democratic Senator Ron Wyden and a coalition of civil-liberties groups, the House lawmakers said they would tolerate any one-time privacy concerns prompted by conducting such a survey because of the importance of the information.

Clapper also said Snowden’s disclosures accelerated the onset of strong commercial encryption available to consumers by seven years, according to analysis from the NSA, which has made it harder to “catch terrorists.”

The issue of encryption has been central to the feud between Apple and the FBI over unlocking iPhones belonging to criminal suspects.