Syrian rebels, including the Nusra Front, seized control of the Syrian crossing with the Israeli-occupied Golan Heights on Wednesday, a pro-opposition monitoring group said.
"Al-Nusra Front and other rebel groups took the Quneitra crossing, and heavy fighting with the Syrian army is continuing in the surrounding area," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights.
He said at least 20 soldiers and four rebels were killed in the fighting for the crossing that began earlier on Wednesday.
The heavy fire strayed across the border into the Israeli-occupied portion of the Golan, where the Israeli army said one of its officers was moderately wounded.
Six mortar shells crashed into the occupied portion of the Golan within hours, the army said, adding that it returned fire.
"In response to the errant fire from the internal fighting in Syria, which hit Israel earlier today and injured an IDF (Israel Occupation Forces) officer, the IDF just targeted two Syrian army positions in the Syrian Golan Heights. Hits were confirmed," it said in a statement.
Israel, which is technically at war with Syria, seized 1,200 square kilometers (460 sq miles) of the Golan Heights during the 1967 Six-Day War and later annexed it in a move never recognized by the international community.
Since the Syrian conflict erupted in 2011, the plateau has been tense, with a growing number of projectiles, mostly stray, hitting the Israeli side, prompting occasional armed responses.
In June, an Israeli teenager was killed in a deliberate cross-border attack, which prompted Israeli warplanes to attack Syrian military headquarters and positions.
Last year, the Quneitra crossing briefly fell to rebel forces in June, before being recaptured by Syrian army troops.