Big US technology firms joined a coalition of activists Wednesday urging Congress to pass a law scaling back government surveillance ahead of key deadline.

A letter endorsed by more than 40 groups including an alliance that includes Google, Facebook, Twitter, and Apple pressed for surveillance reforms ahead of the June 1 expiration of a law allowing bulk collection of telephone records and other data.

"There must be a clear, strong, and effective end to bulk collection practices under the USA Patriot Act," the letter addressed to key lawmakers and administration officials said.

Legislation should "contain transparency and accountability mechanisms for both government and company reporting as well as an appropriate declassification regime for Foreign Intelligence Surveillance Court decisions."

The latest push comes with the key element of the Patriot Act -- known as Section 215 -- set to expire, leaving the future of US surveillance programs unclear.

The White House said this week it would stop bulk collection after June 1 if not specifically authorized by Congress.

But many activists argue there could be other loopholes to allow mass surveillance to continue.

"It is unclear what would happen in a future administration and it is unclear if a future administration would have the power to restart a bulk collection program," said Harley Geiger at the Center for Democracy and Technology, one of the groups endorsing the letter.

Geiger said that certain kinds of mass surveillance could potentially continue under the authority of other provisions of the Patriot Act, and some existing investigations might continue even if the law expires.

- Boosting transparency -

With Congress facing this deadline, Geiger said there is "an opportunity not just to end bulk surveillance but also enhance transparency and make reforms to the FISA court," the secret tribunal which reviews US government surveillance.

The massive surveillance programs have been hotly debated since the leaking of documents by former National Security Agency contractor Edward Snowden in 2013.

Congress last year came close to passing a surveillance reform bill, which failed narrowly in the Senate before the end of the congressional session.

Kevin Bankston at the New America Foundation's Open Technology Institute said the coalition is seeking "an effective ban on the bulk collection of records, the transparency and accountability tools necessary to make sure that ban works."

The group also sees "no new government mandates telling companies what customer data to store or how long to store it, or demanding that companies design their products to make them less secure," Bankston said in a statement.

Those endorsing the letter include the Reform Government Surveillance coalition which includes Google, Yahoo, Microsoft, Apple, Twitter and Facebook.

Other signatories include the American Civil Liberties Union, American Library Association, Human Rights Watch, the Internet Association, Mozilla and the World Press Freedom Committee.