The biggest hurdle to winning a cease-fire in the Gaza Strip, according to diplomats and Israeli military officials, is a problem that has bedeviled Israel for years: how to stop Hamas from digging tunnels into Egypt in order to bring tons of rockets and other weaponry into Gaza.

Mediators are trying to come up with an anti-tunnel plan to satisfy Israel, which has said it won't agree to a truce unless it includes concrete measures to prevent Hamas from rearming. Some of the ideas under consideration include construction of a giant underground barrier along the nine-mile border between southern Gaza and Egypt, as well as international military patrols with the authority to search for and destroy any freshly built tunnels, Israeli officials said.

Former British prime minister Tony Blair, who along with several other European leaders has been trying to broker a deal, said Tuesday in Jerusalem that an "immediate cease-fire" was within reach if a solution to the tunnel problem could be found.

"These circumstances focus very much around clear action to cut off the supply of arms and money through the tunnels that go from Egypt into Gaza," Blair told BBC radio. "I think that is the one basis on which we could bring a quick halt to" the fighting, he added. "Otherwise, I think we are in for a protracted campaign."

Israeli military officials estimated that they had blown up about half of the estimated 300 smugglers' tunnels along the Gaza-Egyptian border since Israel began airstrikes Dec. 27.

Maj. Avital Leibovich, an Israeli military spokeswoman, said that Hamas had used the tunnels to acquire 100 tons of explosives in the past year, among other supplies. "They basically smuggle everything from people to rockets," she said. Israel has imposed an economic blockade on Gaza since Hamas took exclusive control of the territory in June 2007, and Gazans have used the tunnels as their only means of trade with the outside world.

Israeli leaders acknowledged that the Gaza military campaign would serve only as a short-term fix and that Hamas would probably dig a new network of tunnels as soon as the Israeli military withdraws. As a result, they said, any cease-fire deal would need to include a provision for blockading the Egyptian-Gaza border, above and below ground.

"The result must mean an effective blockading of the Philadelphia route, with supervision and follow-ups," Israeli Prime Minister Ehud Olmert said in an interview with the newspaper Haaretz, referring to the corridor that separates southern Gaza from Egypt.

Israel has effectively sealed off Gaza's eastern and northern borders and closely patrols Gaza's western side along the Mediterranean Sea. But it has accused Egypt of turning a blind eye to the tunnels in the south, even though the 1978 peace accord between Israel and Egypt limits the security forces that each country can deploy along their shared border.

Israel's military had warned that smuggling would become a problem before it withdrew its soldiers and settlers from Gaza in 2005 and considered a variety of schemes to thwart potential tunnel diggers. The most audacious idea: an 80-foot-deep moat filled with seawater, with an estimated price tag of $250 million.

But the military nixed the moat proposal after Israel's attorney general said he would oppose it because of fears it would contaminate Gaza's scarce underground water supplies. A plan to dig a giant dry trench was also shelved because it would have required the destruction of hundreds of Palestinian homes along the border corridor.

Those and other plans to construct a subterranean barrier have been getting another look since Israel began its offensive in Gaza last month, officials said.

Roni Bart, a retired Israeli army colonel and a research associate at the Institute for National Security Studies at Tel Aviv University, said a Gaza barrier would cost far less and probably generate much less political controversy than the 456-mile-long security barrier -- which includes fences, roads and walls -- that Israel is building around much of the West Bank.

"It's nothing compared to that," he said. "Anything you can do to stop smuggling, by definition, weakens Hamas."

A senior Israeli official who briefed reporters on the military campaign did not rule out the possibility that Israel would attempt to construct an underground physical barrier along the Gaza-Egypt border.

"I cannot tell you the technical solutions" we are considering, the official said. "But this has to be taken care of and we have a better chance to do it now."

Mark Regev, a spokesman for Olmert, said Israel was open to any plans that would block Hamas from digging more tunnels.

"There are different ideas out there and the Israeli government is willing to hear the input of the international community," he said. "This is a crucial issue for Israel. Without some sort of effective mechanism to stop the flow of armaments, there can be no sustainable calm in the south."

Israeli officials and analysts said any cease-fire deal would hinge on cooperation from Egypt, as would construction of an underground barrier.

Israeli leaders have rejected cease-fire proposals that would involve sending international observers to Gaza, saying that such teams would be ineffective unless they had the authority to destroy tunnels or engage Hamas fighters.

"It's clear that we don't need monitors to tell us that 'today, 10 tons of armaments passed through the tunnels,' " Regev said. "We need a mechanism that will work, but what form that might take is fluid."

Egyptian officials haven't said whether they would allow foreign troops or monitors on their side of the border. But they have held talks in recent days with European diplomats and Hamas delegates.

Hamas has said that it will agree to a cease-fire only if Israel agrees to reopen border crossings from Gaza and end the blockade.

Officials with the Palestinian Authority, which holds power in the West Bank and is led by political rivals of Hamas, said they favor allowing international observers into Gaza as part of a truce. But they have been reluctant to endorse a plan that would give outsiders police powers and the authority to destroy tunnels.

"If you are talking about observers that will go and shut down tunnels, they are not really observers," Riad Malki, the Palestinian Authority foreign minister, told reporters Monday at the United Nations in New York.

Efraim Inbar, director of the Begin-Sadat Center for Strategic Studies at Bar-Ilan University, predicted that a solution would be elusive. He said few countries would be willing to send armed forces to Gaza. He also derided the idea of building a trench or underground wall, saying that smugglers would inevitably find a way around.

"It's just not practical," he said. "It would cost a lot of money. Who's going to pay for it? America?"

He said the most effective course would be to pressure Egypt -- which sees Hamas, an Islamist movement, as a political threat to its secular government -- to take action on its own against smugglers. "That's the only practical way," he said. "All the rest is wishful thinking."