WASHINGTON, United States - CIA chief John Brennan warned Sunday that allowing vital surveillance programs to expire could increase terror threats, as the US Senate convened for a crunch debate on whether to renew the controversial provisions.

With key counterterrorism programs set to expire at midnight Sunday, the top intelligence official made a final pitch to senators, arguing that the bulk data collection of telephone records of millions of Americans unconnected to terrorism has not abused civil liberties and only serves to safeguard citizens.

"This is something that we can't afford to do right now," Brennan said of allowing the expiration of counterterrorism provisions, which "sunset" at the end of May 31.

"Because if you look at the horrific terrorist attacks and violence being perpetrated around the globe, we need to keep our country safe, and our oceans are not keeping us safe the way they did century ago," he said CBS' "Face the Nation" talk show.

Brennan added that groups like Islamic State have followed the developments "very carefully" and are "looking for the seams to operate."

The House has already passed a reform bill, the USA Freedom Act, that would end the telephone data dragnet by the National Security Agency and require a court order for the NSA to access specific records from the vast data base retained by telecommunications companies.

If no action is taken by the Senate Sunday, authorities will be forced to shut down the bulk collection program and two other provisions, which allow roving wiretaps of terror suspects who change their mobile phone numbers and the tracking of lone-wolf suspects.

Senator Rand Paul, a Republican 2016 presidential candidate adamantly opposed to reauthorizing the surveillance, is threatening to delay votes on the reform bill or an extension of the original USA Patriot Act.

That would force the counterterrorism provisions to lapse until at least Wednesday.

Former NSA chief Michael Hayden, who is also a former CIA director, equated such a temporary lapse as "giving up threads" in a broader protective fabric.

"It may not make a difference for a while. Then again, it might," he told CNN's State of the Union.

"Over the longer term, I'm willing to wager, it will indeed make a difference."