Ben Emmerson, U.N. special rapporteur on human rights and counter-terrorism, also said he was concerned combat drones, currently largely used by the United States, could fall into the hands of terrorist groups.
"The plain fact is that this technology is here to stay, and its use ... is a reality with which the world must contend," Emmerson told reporters in London.
The increasing availability of drone aircraft, controlled by onboard computers or under the remote control of pilots elsewhere, "makes it very likely that more states will be using this technology in the coming months and years," he said.
Its growing use also "includes raising the specter that non-state organizations -- organizations labeled as terrorist groups -- could use the technology in retaliation," Emmerson told reporters.
He called the threat of widespread use of drones, or unmanned aerial vehicles, a "very serious and escalating situation" that the international community must address "urgently."
The rapporteur's probe, undertaken at the request of Russia, China and Pakistan, will provide a "critical examination of the factual evidence concerning civilian casualties" from drone attacks and make recommendations to the U.N. General Assembly about "the lawfulness and proportionality of such attacks," the U.N. Office of the High Commissioner of Human Rights said in a statement.
"Rapporteur" is a French-derived word for an investigator who reports to a deliberative body.
The probe will examine 25 attacks in Pakistan, Yemen, Somalia, Afghanistan and the Palestinian territories, Emmerson said.
"This is not an investigation into the conduct of any particular state. It's an investigation into the consequence into this form of technology," Emmerson told al-Jazeera.
U.S. officials defend drone strikes as a justified use of force against al-Qaida and its allies. They say such attacks have spared pilots' lives and forestalled deeper military involvement abroad.
Such strikes have killed key militants, including No. 2 al-Qaida leader Abu Yahya al-Libi, killed in Pakistan's Federally Administered Tribal Areas June 4, 2012.
But the rising drone use alarms human rights groups that say the secretive practice of targeted killings goes against international law, the Los Angeles Times said.
Emmerson said he already received a promise from Britain to cooperate with his investigation and said he hoped the United States would do the same.
The Obama administration, which has intensified drone use, had no immediate comment.
Emmerson said he already asked Washington to release "before and after" videos of combat drone strikes and internal reports of those killed, including civilians.
He said he planned to make his recommendations to the General Assembly in October.