Reporting from Tel Aviv— The aftermath of the "Arab Spring" is forcing Israel to gamble with what had long been one of the foundations of its security: a demilitarized Sinai peninsula.

The agreement to bar Egyptian soldiers from the Sinai border was a linchpin of the landmark 1979 Egyptian-Israel peace treaty, which returned the desert region to Egyptian control. But an increase in violence since January, culminating in a cross-border attack this month that left eight Israelis dead, has led Israel this year to reluctantly allow the temporary deployment of several thousand Egyptian soldiers to the peninsula.

This week, as many as 1,500 more Egyptian troops poured into the region with armored vehicles and a limited number of tanks amid a crackdown on Islamist radical groups suspected of plotting another attack.

Israel is hoping that the additional troops will help Egypt's interim government regain control of the Sinai, which descended into lawlessness in February when local police fled their posts after the collapse of former President Hosni Mubarak's regime.

But allowing the heaviest Egyptian military presence along the border with Sinai since the 1967 Middle East War carries significant risk for Israel, particularly as its relations with Egypt look more fragile than at any time in three decades.

Israel had a reliable ally in Mubarak. But the military-led council that replaced him is facing strong public pressure to take a harder stance against Israel, which remains deeply unpopular in Egypt. After three Egyptian soldiers were killed during an Israeli military incursion into the Sinai this month, Egypt threatened to recall its ambassador until Israel formally apologized for the incident.

"Egypt is now a different Egypt," said a senior Israeli Defense Ministry official, who requested anonymity in keeping with Israeli policy.

The official said Israel has agreed since January to permit Egypt to deploy "several thousand" soldiers along the Sinai border. He declined to give exact figures, but estimated the number at fewer than 5,000.

However, he said, Israel so far has been unimpressed with the Egyptian army's results.

"They can't or won't clamp down to stop the weapons flow," he said, and in recent months Libyan-made shoulder-launched missiles and antitank missiles have been smuggled through Sinai into the Gaza Strip.

He said Egypt has the manpower to accomplish the job, if it has the will.

"The question of force level is secondary," he said. "It depends on what you do with your forces.... They can do more, but they are under immense constraints from their own public."

He said Egypt should redeploy police officers, set up checkpoints on key roads, destroy smuggling tunnels into Gaza and begin developing the infrastructure and economy to assist the Sinai's marginalized communities.

Adding to Israel's conflicted feelings about allowing the new troop levels are suspicions that Egyptian security forces may have played a role in the Aug. 18 attack in southern Israel. Some of the gunmen killed by Israeli forces were Egyptian, he said. Though Israel believes the attacks were masterminded by the Gaza-based militant group Popular Resistance Committees, it is investigating the possibility of an Egyptian security forces role.

In recent days, the Israeli official acknowledged, Egypt has demonstrated a stronger determination to rein in criminal gangs and Islamic terrorist groups believed to be operating in the Sinai.

But the risk for Israel, analysts say, is in opening a door that will be difficult to close again. Once Egyptian soldiers are deployed in the Sinai in large numbers, can Israel be assured that they will leave after the threat is controlled?

Already there are calls in Egypt to renegotiate the 1979 accord, which permits police only to patrol the border. The restriction, which Israel agreed to ease to combat the current violence, has long been a source of embarrassment to Egyptians and many are pushing for a permanent military presence in the Sinai, something many Israelis would oppose.

"The risk is that slowly and progressively the Egyptian army gets back in and the Sinai is remilitarized," said Zvi Mazel, a former Israeli ambassador to Egypt. "But I don't see that happening. People in Israel don't want to see Egyptian soldiers just a few kilometers from Ashdod," he added, referring to Israel's main port.

He said that cooperation and coordination between the countries' militaries remained strong, but that the wild card was the recent rise in anti-Israel sentiment in Egypt and the effect it could have on the interim government's decisions.

Others argue that the Egyptians cannot be expected to control the Sinai — a region that Israel now believes has been infiltrated by Al Qaeda-affiliated groups — with lightly armed border police. They say the threat to Israel from Sinai-based terrorism is greater than the risk of ground attack by Egypt.

"Israel's limitations on the Egyptian military presence in the Sinai could also be to Israel's detriment," said an editorial Monday in the Israeli newspaper Haaretz.

The question has led to a lot of hand-wringing recently among Israeli politicians. Lawmakers have insisted that any significant changes to the treaty be approved by the Knesset, Israel's parliament. Prime Minister Benjamin Netanyahu said Sunday that there was no need to "rush into" modifications of the treaty. Defense Minister Ehud Barak said no additional Egyptian troops, beyond those already agreed upon, were expected to be approved.

Instead, Israeli military officials said they were reviewing their security arrangements along the Sinai border, which was once viewed as one of Israel's safest. Israel is accelerating efforts to construct a fence, which is expected to be completed next year. They have also deployed anti-terrorism units to the area and are attempting to improve intelligence capabilities.

Assumptions that the Egyptian border would remain quiet and stable have been tossed aside, the senior Israeli Defense Ministry official said.

"You need to recheck your basic assumptions," he said. Eight months ago, Israel assumed that "Egypt is behind us, but safe and not moving," he said. "Now we need to spend more time looking in the rear-view mirror."