BEIRUT — Three Syrian tanks entered the demilitarized zone in the Golan Heights on Saturday, Israel said, raising concerns that violence from Syria’s civil war could heat up a long-quiet frontier that has not seen such an incursion in nearly 40 years.

Israel complained to UN peacekeepers present in the area, a relatively low-key response that suggested it did not see the Syrian armor as an immediate threat. But the entry marks the most serious spillover of Syria’s turmoil to date at the frontier, where stray ordnance has exploded on the Israeli side in the past.