A top Senate Democrat has "serious questions" about whether President Obama is violating international law by carrying out some especially controversial drone strikes.

"The alleged use of signature strikes, they raise a very serious question of whether [the drone strikes] comply with the international humanitarian law we joined in," Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., said Thursday during the Human Rights First Summit in Washington.

An attempt to ban "signature strikes" -- "attacks in which the targets of lethal U.S. actions are identified by their actions rather than as individuals," per Politico -- failed in the House Intelligence Committee earlier this year.

Leahy stipulated that "drones can be used in an armed conflict but only in accordance with international humanitarian laws." He suggested that the international community should work to limit U.S. use of drones.

"Maybe it's time to look again at international law in this area and maybe it's time to have some tightening of it and some changes in it," Leahy said. "I, for one, would like to see that."