LONDON — Prime Minister David Cameron revealed Monday that British forces had used a drone strike over Syria in August to kill three Islamic State fighters, including two Britons.
He told Parliament that the attack was legally justified because the militants were plotting lethal attacks against Britain and the fighters could not be eliminated any other way.
‘‘There was a terrorist directing murder on our streets and no other means to stop them,’’ Cameron said, adding that the decision to launch the attack hadn’t been taken lightly.
The prime minister said the deadly Royal Air Force strike was permissible because of Britain’s intrinsic right to self-defense and had been approved by the attorney general.
In a separate development, Pakistan said it used its first-ever armed drone on Monday, killing three ‘‘high-profile’’ militants near the Afghan border, according to the army. The missiles hit a compound in the Shawal valley of the Waziristan tribal region, an army statement said.
The British drone attack on Aug. 21 struck a car in the Syrian city of Raqqa, an Islamic State stronghold. The action was an escalation for Britain, which had not participated in military actions in Syria, but Cameron said the threat made action mandatory.
Under questioning by Labor Party interim leader Harriet Harmon, the prime minister said the attack marked the first instance in modern times that a British asset has been used to conduct a strike in a country where Britain was not involved in a war.
Cameron said that after ‘‘meticulous planning,’’ British nationals Reyaad Khan and Ruhul Amin were killed along with another militant who was not identified.
Khan was from Cardiff, Wales, and had taken part in a militant recruiting video.
Cameron said Britain took action after determining that Khan and another Briton identified as Junaid Hussain were ‘‘British nationals based in Syria who were involved in actively recruiting [Islamic State] sympathizers and seeking to orchestrate specific and barbaric attacks against the West, including directing a number of planned terrorist attacks right here in Britain, such as plots to attack high-profile public commemorations, including those taking place this summer.’’
He said it was their intention to murder British citizens. There was no information indicating that Hussain, the other British national cited by the prime minister as planning attacks, had been injured or killed.
The prime minister said the threat from Islamic State fighters was more acute than ever and the drone attack was the only ‘‘feasible means’’ of dealing with the danger in this case.
No civilians were killed in the strike, which was carried out independent of coalition military activity taking place in the region, he said.
Parliament was not consulted in advance. Cameron said the government reserves the right to take future action without prior approval when there is a ‘‘critical’’ British interest at stake or when a ‘‘humanitarian catastrophe’’ is imminent and can be averted.
In Pakistan, the army first displayed the drone used in the Waziristan attack in March, saying it had been successfully test fired in targeting both static and moving targets. But little is known of the drone’s armament or capabilities.
Pakistan has sought to develop its own drone program for years and displayed a drone meant solely for surveillance in 2012.
The United States frequently uses drones to target militants in Pakistan’s tribal regions, but it has refused to share the technology with Islamabad — a stance that former prime minister Raja Pervaiz Ashraf once called ‘‘undeclared technological apartheid.’’
The US drone strikes in Pakistan have been a frequent source of tension and controversy; they are unpopular among many Pakistanis and the government has occasionally decried them as violations of Pakistani sovereignty.