An Islamic State operative suspected of helping plan the Paris attacks had been monitored in Syria by Western allies seeking to kill him in an airstrike, but they couldn’t locate him in the weeks before the plot was carried out, two Western security officials said.

The operative, a Belgian citizen named Abdelhamid Abaaoud, was convicted in absentia in Brussels earlier this year of recruiting jihadists, was suspected of masterminding a foiled plot to behead police officers, escaped to Syria and was profiled in Islamic State’s...