The top Republican on the Senate Intelligence Committee said Sunday 21 U.S. embassies across the Middle East and North Africa were closed as a direct result of increased “chatter” among terrorist organizations that is “very reminiscent of what we saw pre-9/11.”

Sen. Saxby Chambliss, R-Ga., said on NBC’s “Meet the Press” that discussions between terrorist groups intercepted by U.S. intelligence pose the “most serious threat I’ve seen in a number of years.”

The State Department issued a worldwide travel alert Friday warning Americans abroad that there is a significant threat of a terrorist attack through the end of the month. North Africa and the Middle East are especially on watch, and embassies in those regions have been closed down. Both steps are rarely used measures that suggest the White House and the intelligence community see threats as credible.

Chambliss said the intelligence gathered that led to the closures was the direct result of a controversial National Security Agency program that allows the U.S. to listen in on phone calls, read emails and gather other electronic evidence from virtually anyone.

“They are what lead us or allow us to have the ability to gather this chatter that I referred to,” Chambliss said. “These programs, even though they’re controversial, this is the reason they’re so important.”

A number of congressmen on both sides of the aisle and either end of the Capitol have backed efforts to would curb the NSA’s ability to listen in.

Chambliss and Sen. Dick Durbin, D-Ill., spent more than 90 minutes with President Obama discussing the program, Durbin said, and were briefed by Vice President Joe Biden last week on the most recent intelligence leading them to believe a terrorist attack could be imminent.

Durbin, the No. 2 Democrat in the Senate, also said he was open to reforming the NSA program to protect the privacy of American citizens, but stressed that it is also a critical tool in fighting terrorism.

“We need to know and realize we are living in an increasingly dangerous world,” Durbin said.