The US government has defended its use of drone strikes in Pakistan, Yemen and other countries in front of the UN, telling a chamber full of largely critical nations that in President Obama's view the deployment of unmanned aerial attacks against al-Qaida targets was "necessary, legal and just".

Representatives from a slew of nations, including Brazil, China and Venezuela, lined up to berate the Obama administration for its intensive use of drone strikes. But the US delegation told a plenary meeting of the general assembly in the UN building in New York the president had taken steps to introduce new guidance and standards, and to set out the legal rationale for unmanned weapons deployed in the fight against al-Qaida and affiliated threats.

The UN debate marked the first time that member nations have come together to discuss the rapidly expanding militarised use of remotely piloted aircraft and the fraught international legal issues that it raises. It came at the climax of 10 days in which the question of the legality of drones has caught the headlines, with the release of two UN reports that have sharply condemned aspects of the programmes.

The authors of the two reports addressed Friday's UN debate, beginning with Christof Heyns, the UN's special rapporteur on extrajudicial, summary or arbitrary executions. His study warned of the danger of proliferation of the un-piloted weapons among states and terrorist groups.

In his opening remarks to the UN debate, Heyns said "drones are here to stay". He argued that it was hard to make a case that unmanned aircraft were inherently illegal: "It is difficult to suggest that a weapon system is unlawful because a pilot is not on board."

But he added that drones were easy to deploy across international borders, often secretly. "So it is my view that although they are not illegal, they do pose a challenge, particularly as they are used often in secret, raising accountability issues."

The accountability theme was picked up by the second UN expert, Ben Emmerson, the special rapporteur on human rights and counter-terrorism. His ongoing investigation into lethal extra-territorial counter-terrorism operations has concluded that the 33 drone strikes that are known to have caused civilian casualties may have been carried out in violation of international law.

He told the assembled nations that lack of transparency was "the single greatest obstacle to an evaluation of the civilian impact of drone strikes and it's a challenge which makes it extremely difficult to assess claims of precision targeting objectively".

He urged delegates to consider whether an absolute ban was needed on secret deals cooked up between states for engagement in joint military actions.

Emmerson also underlined the state of chaos that exists in international law over drones: "Despite the proliferation of this technology, there remains a lack of consensus among international lawyers and between states on the core legal principles."

He added: "It's not the drone that is the problem. The problem is the lack of clarity under which it is lawful to deploy lethal force by drone."

The UK, one of three countries alongside the US and Israel that have attracted most attention for the use of unmanned assault aircraft, also tried to defend its military deployment of the technology. Emmerson's report points out that the UK government has reported only one incident involving civilian casualties: an RAF strike in March 2011 in Afghanistan in which four civilians were killed.

The UK mission attempted to defend its use of drones in military situations, telling the debate that the weapons systems were controlled by personnel on the ground and were therefore not "autonomous" or robotic – a status which would push them over into illegality in the opinions of most legal experts. The UK has no plans to replace controlled drones with autonomous weapons, the UN was told, while the engagement of unmanned planes within the RAF falls under exactly the same strict military rules as ordinary piloted fighter jets.

But several countries questioned the legality of the weapons. Venezuela called drones "flagrantly illegal" and said that by its accounting, 1,800 people had been casualties – only about 10% of whom were "targeted individuals". "This is like a collective punishment," Venezuela's representative said.

Brazil wondered where the line would be drawn in terms of potential targets for drone strikes. "In certain regions we might have sympathisers of terrorists – does that mean they become 'fair game' just because they sympathise with a particular cause, that they are legitimate targets of drone attacks, for yet another kill? This is uncharted waters."

China, which normally keeps to the sidelines of the most contentious international disputes, was driven to state that drones were a "blank space in international law, and this blank space is subject to abuse … We should respect the principles of UN charters, the sovereignty of states and the legitimate rights of the citizens of all countries."