Image caption Drone warfare has become common in the US pursuit of al-Qaeda and the Taliban

The US has defended its drone strikes in Yemen and Pakistan, saying it takes "extraordinary care" to ensure they comply with international law.

The unmanned raids targeting terror suspects were a "course of action least likely to result in the loss of innocent life", the White House said.

It follows allegations by Human Rights Watch and Amnesty International of the unlawful killing of civilians.

Pakistan's PM on Tuesday urged the US to end drone attacks in his country.

Speaking at the start of a visit to the US, Nawaz Sharif said the attacks violated his country's sovereignty.

He added that the raids were a "major irritant" in relations with Washington.

Drone warfare has become common in the US pursuit of al-Qaeda and the Taliban.

Few details are known about these covert operations, which are directed remotely from control rooms, often on other continents.

Analysis The new Amnesty International report has sought to document more evidence of civilian deaths in drone strikes than before, but some of the cases they have brought forth as examples were already known to the media. As for local claims of civilian deaths in strikes targeting militant camps, they are almost impossible to prove. One reason is the restricted media access in the region. The other is the militants' tendency to cordon off the targeted sites and make quick burials. But Waziristan is a small place, with well-knit tribal clans living and interacting closely with each other. Civilian deaths cannot go unnoticed here, and anecdotal evidence travels fast. The general impression that one gets from talking to elders and correspondents from the area is that drone strikes are for the most part accurate, causing little or no collateral damage. They say if civilians deaths had been as high as those mentioned in some recent international reports, there would have been more of an outcry against it both socially and also in the media.

Senior militant leaders have been killed but civilians have also died, causing outrage in Pakistan, where many assert that the strikes cause indiscriminate deaths and injuries.

'Strongly disagree'

In a new report released on Tuesday, Amnesty said it reviewed nine of 45 recent drone strikes in North Waziristan and found a number of victims were unarmed.

In a separate report looking at six US attacks in Yemen, Human Rights Watch says two of them killed civilians at random, violating international law.

White House spokesman Jay Carney said Washington was "reviewing the reports carefully".

"To the extent these reports claim that the US has acted contrary to international law, we would strongly disagree.

"The administration has repeatedly emphasised the extraordinary care that we take to make sure counter-terrorism actions are in accordance with all applicable law," he said.

In the report, Will I Be Next? US Drone Strikes in Pakistan, Amnesty called on the US to disclose information and the legal basis for strikes carried out in Pakistan.

It said US President Barack Obama's pledge earlier this year to increase transparency around drone strikes had not been fulfilled.

It called on the governments of Pakistan, Australia, Germany and the UK to investigate drone strikes or other abuses that may constitute human rights violations.

Meanwhile, Human Rights Watch said in its report that six US drone attacks in Yemen had killed 82 people, including at least 57 civilians.

It added that two of the strikes killed innocent people indiscriminately.

Last week, a UN investigation found that US drone strikes had killed at least 400 civilians in Pakistan, far more than the US has ever acknowledged.

UN special rapporteur Ben Emmerson accused the US of challenging international legal norms by advocating the use of lethal force outside war zones.

A controversial aspect of the US policy is that drone attacks are carried out not by the military but by the Central Intelligence Agency.

US Defence Secretary Leon Panetta has argued in favour of the policy, saying that the US will continue to defend itself.

President Obama has insisted the strategy was "kept on a very tight leash" and that without the drones, the US would have had to resort to "more intrusive military action".