The US director of intelligence has said wiretaps played a significant role in stopping bomb attacks by suspected Islamists in Germany last week.
Three men were arrested for planning an attack on US facilities
Michael McConnell told a Senate committee eavesdropping had revealed that the suspects had obtained explosive liquids.
He said Congress should not restrict the programme.
In August, a temporary bill was adopted allowing eavesdropping on foreign terror suspects without a warrant.
Mr McConnell said the surveillance programme had made "significant contributions" in discovering and breaking up a suspected plot in Germany to bomb US installations.
Michael McConnell pleaded against the restrictions
"It allowed us to see and understand all the connections to al-Qaeda," Mr McConnell told a hearing of the Senate Homeland Security Committee.
The programme also contributed to the arrests in Denmark of eight Muslims, with suspected links to al-Qaeda, on suspicion of planning a bomb attack, National Counterterrorism Centre Director John Redd told reporters later.
Civil liberties advocates have criticised portions of the programme which allow the monitoring of international calls to people in the US.
Congress passed legislation in August easing for six months restrictions on wiretapping under the Foreign Intelligence Surveillance Act.
Some Democrats have vowed to revise it as early as possible.