LONDON — Islamic opposition fighters in Syria, including members of an al-Qaida affiliate, took control of the Quneitra crossing point on the demarcation line with the Israeli-occupied Golan Heights, activists said Wednesday.

The move could bring militant forces within 200 yards of territory controlled by Israel. An activist in the area, contacted by Skype, said a coalition of rebel groups, including members of the al-Qaida-linked Nusra Front, opened an assault on the government-held crossing early Wednesday. The status of a U.N. force that is supposed to monitor the crossing point was unclear.

Militants with a rival and more extreme Sunni militant group, the Islamic State, have spread from Syria and now threaten northern and central Iraq.

The Israeli military said one soldier and an Israeli civilian were wounded by “errant fire” from the clashes at Quneitra on Wednesday, prompting an artillery barrage against two Syrian army positions in the Golan Heights, the latest of several occasions when Syria’s civil war has spilled into Israel, prompting retaliation. Israel has said it has no interest in further involvement in the fighting.

Israeli troops saw large plumes of smoke as gunfire and explosions rattled the area, news reports said.

Yaakov Amidror, a former Israeli national-security adviser, said reports of the Nusra Front taking control of the Quneitra crossing could be “very significant” if the group managed to link that position to its stronghold in Daraa, in southern Syria, and other areas.

But Amidror, who served under Prime Minister Benjamin Netanyahu, said Israel should not interfere in the conflict and should respond only if attacked or to provide humanitarian assistance to wounded people on the demarcation line.

The Syrian Observatory for Human Rights, an activist group based in Britain that gathers information from contacts inside Syria, said the fighting killed at least 20 government soldiers and an unknown number of the attackers.

Syria lost the strategic Golan Heights to Israel in the 1967 war. Israel later annexed the area, but left Quneitra. It has since been depicted by the authorities in Damascus as an emblem of Israeli expansion and remains in ruins.

Although the town has little strategic value, insurgents have been fighting to drive off government forces for more than a year, said Abu Mossab, another activist in the region.

Accounts of the fighting were unclear, but some activists said members of the Western-backed and secular Free Syrian Army were also involved in the assault.