Estimates: More drone attacks in September than any other month

The potential plot against Europe was one factor contributing to the uptick this month in missile strikes by unmanned drones against terrorist targets in Pakistan, according to a U.S. official.

"We would be remiss not to try to take action to thwart what might be under way in Europe," said the official.

The official emphasized that the potential plot was not the sole factor.

U.S. officials say they are taking advantage of what they call "precise intelligence."

Most of the drone attacks this year have been focused on North Waziristan, a mountainous area bordering Afghanistan where Pakistani security forces have little control. That has continued to be the pattern this month.

Based on information from Pakistani officials, CNN estimates there have been 20 drone attacks in the area in September alone, a higher number than in any previous month, and more than twice the monthly average.

Acknowledging the spike, one U.S. official told CNN: "Our operational tempo has been up for a while now, we have good information driving it, and given the stakes involved, we hope to keep the pressure on as long as we can."

According to the official, the mix of threats remains the same. It comes from groups like the Haqqani network, al Qaeda, the Afghan Taliban and the Pakistan Taliban. The threats they pose are "all deadly," said the official.

Pakistani officials say many recent strikes have been aimed at compounds in or around the town of Miramshah, a stronghold of the Haqqani network.

Western intelligence officials have long regarded the Haqqanis as one of the most dangerous terror groups and have linked them to several attacks in Kabul.

Intelligence analysts point to other reasons for the escalated drone attacks.

They include better information from sources in the border area and better surveillance technology - including the growing use of spy balloons fitted with high-powered cameras.

In addition, the rising number of drone strikes is designed to deprive the Afghan Taliban of "strategic depth" as the Obama administration's campaign to defeat the insurgency enters a crucial phase and tighten the noose on the senior al Qaeda leadership.

Pakistani officials say one strike last weekend killed Sheikh Mohammad Fateh al Masri, described as the group's senior operational commander.