After new clashes between Syrian troops and rebels in the Golan, and Austria's announced withdrawal from a UN monitoring force, Israel is concerned about security along its ceasefire line with Syria.
The army remained on high alert in the region on Friday, bringing up reinforcements of tanks and troops, and readying an anti-tank missile unit, media said.
That came after Austria said it would withdraw peacekeepers from the UN force, citing deteriorating security, and the Philippines said it was studying a similar move after one of its soldiers was wounded in the fierce fighting on Thursday.
The Quneitra crossing between the Israeli and Syrian sides of the armistice line was briefly seized by rebels and then recaptured by government troops following small-arms and artillery exchanges, a security source said.
An AFP correspondent saw tanks of President Bashar al-Assad's regime approaching the ceasefire line later in the day.
Israeli army spokesman Captain Arye Shalicar told AFP on Thursday: "We're watching very closely what's happening there, and we have to be ready for any development. We hope there won't be any spillover into Israel.
"It is very worrying because, on the one hand, you have jihadists and Islamists who are fighting there (on the rebel side) and, on the other hand, you also have government forces which are allied with (the Lebanese militant group) Hezbollah.
"We certainly don't want to have Hezbollah on two fronts," he said, referring to the presence of the group in southern Lebanon, next to the Israeli border.
Quneitra, the only crossing along the ceasefire line, is used almost exclusively by Druze residents of the Israeli-occupied Golan Heights who are allowed to cross over to study, work or get married.
Several dozen Syrian civilians tried to enter Israel through Quneitra on Thursday but were turned away by the Israeli army.
The crossing remained closed on Friday.
The Israeli army has repeatedly warned farmers on the Syrian side against straying too near the ceasefire line.
Because of the fighting, the stability and makeup of the United Nations Disengagement Observer Force, which has monitored a ceasefire between Israel and Syria since 1974, has come into question.
Austria, which has been a cornerstone of UNDOF, said on Thursday it would withdraw its 377 soldiers, who make up more than a third of the force, because the threat to them had "reached an unacceptable level."
"The secured movement and supply of our troops on the Golan can no longer be guaranteed," added a joint statement from Chancellor Werner Faymann and Vice Chancellor Michael Spindelegger.
Other contributors to the mission include India, Morocco and Moldova.
In Manila, the government said on Friday it was studying whether to pull its 341 troops out after a Filipino soldier was wounded by shrapnel from the fighting on Thursday.
An Indian soldier was also lightly injured on Thursday.
Israel reacted with dismay to the Austrian announcement, as the UN held emergency talks to replace its contingent.
"In light of Austria's announcement, there are questions over the stability of the 'border' between Israel and Syria," said Jacques Neriah, an Israeli reserves colonel and former military intelligence analyst.
He warned that the Filipinos "could follow the Austrians."
"The Security Council should therefore present a solution. And (although) Syria has no interest in having tension with Israel... we cannot leave the zone without observers," he said.
In Moscow, President Vladimir Putin proposed on Friday that Russian peacekeepers replace the departing Austrians.
"Considering the difficult situation that is developing today in the Golan Heights, we could replace the departing Austrian contingent in this region separating Israeli forces from the Syrian army," Russian news agencies quoted Putin as saying.
The Kremlin chief stressed that this was only a proposal and that a formal request had to come from the United Nations and be agreed with both Israel and Syria.