Covert terror strikes by United States' drones along the Pakistan border is becoming a common feature of the war against terror, says a report by the Center for American Progress.

Initiated by the Bush administration, the strikes have continued after Barack Obama [Images] took over as the US President, with two strikes occurring just three days after the president's inauguration, says the report.

The strikes have been condemned by the people of Pakistan and its government. However, a recent report suggested that the Central Intelligence Agency was using a deserted airbase in Baluchistan to launch these strikes, indicating the complicity of Pakistan's intelligence agencies.

The regrouping of Taliban [Images] militants and Al Qaeda [Images] factions, the inhospitable terrain of the Federally Administered Tribal Areas and the Pakistan government's lax administration in the region has prompted the US to launch these air strikes, says the report.

For this mission -- the US has chosen remotely directed unmanned aerial vehicles -- the Predator being the most favoured missile.

The Predators were first introduced in Afghanistan, says the report, and was later used to target Qaeda bases in Pakistan's border areas.

At least 45 covert strikes, launched by US, have taken place inside Pakistan since March 2009. US forces had even launched a ground raid in September 2008, but dropped the idea after vehement opposition from the Pakistan government, the report claims.

It is difficult to analyse the impact of these top-secret strikes that are carried out in remote areas, but Center for American Progress analysis of available open-source reporting states that between 402 and 543 people may have been killed in these strikes.

It is not possible to ascertain how many of the victims were Taliban or Qaeda fighters and how many were civilian casualties.

Though the strikes have succeeded in eliminating many top operatives of the Al Qeda network, public sentiment against such strikes are extremely strong in Pakistan, and top officials have often termed then an infringement on the country's sovereignty, stated the report.

But many believe that US and Pakistani intelligence agencies are working together in pinpointing the locations of the targets and then carrying out the strikes.

"Protests are really for the sake of public opinion. These operations are helping both sides. We are partners on this," the report quotes a Pakistani official as saying.

With Taliban forces taking over Swat valley and posing a serious threat to Pakistan, the covert strikes to eliminate extremist elements is likely to continue, in spite of the US administration's silence and the Zardari government's protests on the issue.