The IDF struck targets in Syria Monday night, targeting two Syrian regime artillery batteries near the border, after a shell fired from the Arab’s nation’s territory exploded in Israel.

The apparently errant fire — the fifth such incident in just over a week — came hours after a ceasefire brokered by Russia and the United States took effect in Syria at sundown Monday.

The military noted that it viewed the regime of Bashar Assad responsible for any attack emanating from Syria.

The initial 48-hour truce came into force at 7:00 p.m. local time (1600 GMT) across Syria except in areas held by jihadists like the Islamic State group.

The stray fire was reported by the Israel Defense Forces at around 9 p.m. local time.

As the ceasefire went into effect, Syria’s armed forces immediately announced a seven-day “freeze” on military operations, lasting to midnight next Sunday. But opposition forces had yet to formally sign on and the deal’s fragility was underscored just hours before sundown when Assad vowed to retake the whole country from “terrorists.”

The civil war in Syria has generated a number of spillover incidents over the years. Israel has said it holds the Syrian regime responsible for all errant fire, regardless of the source. The IDF frequently retaliates against stray missile strikes inside Israeli territory.

On Saturday, a mortar shell exploded on the Israeli Golan Heights, prompting an Israeli retaliation. A military spokeswoman said the projectile was most likely unintentional “spillover” from the internal fighting in Syria. A short time later, the Israeli Air Force struck artillery targets belonging to the Syrian army, the army said.

Hours later, a second mortar shell fired from Syria struck the Golan Heights. The shell exploded on the border, causing no casualties or damage.

The Israeli Air Force overnight Wednesday–Thursday hit a number of rocket launchers belonging to the Syrian army after the earlier projectile struck an open area on the Israeli side of the Golan on Wednesday evening. The shell caused no injuries or damage, after reportedly landing in an open field in Kibbutz Merom Golan.

Last Sunday, a mortar round fired from Syria hit an open area on the Israeli side of the Golan demilitarized zone, causing no injuries or damage. In response, the IDF later on Sunday struck Syrian army artillery targets on the plateau.