DOVEV, Israel - From this sunbaked ridge, the small outpost is clearly visible: a trailer that appeared one morning in April, quickly followed by a two-story observation tower, just feet from the hotly contested "Blue Line" that separates Lebanon and Israel.

Lebanon says the structures are used by an environmental group. But Israeli officials say the tower belongs to Hezbollah, the Iranian-backed Lebanese militia group, and is one of 22 outposts that have appeared along the U.N.-monitored Blue Line in the past three months - part of a sudden and worrying escalation that has led Israel to put its northern forces on high alert.

"This is a major change in what we've seen in the last few years," said a senior Israeli military official who spoke on the condition of anonymity to discuss security issues. "Hezbollah is becoming very, very blatant."

The group's activities - which officials say include a doubling of the number and size of patrols near the border, a series of drone incursions and a drumbeat of threats from Hezbollah leaders - come as American mediators race to settle a dispute between the two countries over suddenly lucrative natural gas fields in the Mediterranean Sea.

Israel is already developing one drilling site, the Karish Field, in what Lebanon claims are disputed territorial waters. After two years of talks, time is running out to reach a settlement by September, when Israel is expected to begin extracting gas from the first rig.

Negotiators have indicated that a deal may be close, after recent visits to both Lebanon and Israel by Amos Hochstein, the U.S. senior adviser for energy security. But the stakes are rising.

Hezbollah has threatened to attack Israel if an acceptable deal isn't reached and has dispatched drones toward the gas field at least twice in recent weeks, including three unmanned aircraft that were shot down by Israel in early July.

The drones appeared to be unarmed and caused no damage. But they hinted at Hezbollah's ability to strike the offshore facility at a time when Lebanon's economy is cratering.

"We will reach Karish and everything beyond Karish and everything beyond that," Hezbollah leader Hasan Nasrallah said in a televised speech. "War is much more honorable than the situation Lebanon is heading to now - collapse and starvation."

This month the head of Israel's military advised the security cabinet that the situation was at risk of turning into a military escalation with Hezbollah, according to Israeli media reports. The Israel Defense Forces (IDF) reportedly warned Hezbollah through intermediaries that any attack will provoke fierce retaliation.

"If the deal is not accepted by Lebanon or Israel, we are heading into a confrontation," said Jacques Neria, who was an adviser to Prime Minister Yitzhak Rabin in the 1990s and is now with the Jerusalem Center for Public Affairs. "Any war that starts on the maritime battlefield will spill out onto other arenas. If they hit our rigs, we'll hit them on land."

The spike in tensions comes as Israel nervously eyes international efforts to restore an agreement with Iran over its nuclear program. Israel says a new deal, which would presumably ease financial sanctions against Iran, risks further empowering Tehran and its proxies, including Hezbollah and Palestinian Islamic Jihad, with which the IDF fought a three-day battle this month that killed 45 people in Gaza.

After a breakthrough in talks in Vienna this week, negotiators have returned to their capitals to consider what Europe called its "final" proposal to save the nuclear deal. Washington, over Israel's objections, expressed its support for the text, but Tehran has sought to temper expectations.

For the Israelis and Lebanese living along this hilly border, sometimes within meters of one another, the risk of war is ever-present. In many Israeli communities, it is easy to hear the call to prayer from Lebanese mosques and the firecrackers from Lebanese birthday celebrations.

"They are our neighbors. Right there, those are Lebanese cars," said Silka Schreiber, pointing at vehicles traversing the valley a half-mile from the grocery store in Metula, an Israeli town of fewer than 2,000 bound on two sides by the Blue Line fence.

But the proximity doesn't make the current flare-up seem less dangerous to those living in the line of fire. A 2006 war between Israel and Hezbollah lasted for 34 days, killing more than 1,000 people in Lebanon and dozens of Israelis. Osnat Ben Nun, 55, was pregnant during the tense run-up to the conflict and remembers antiaircraft explosions like fireworks in the night sky. Now, her child is an Israeli soldier.

"My son is in the army," said Ben Nun, a social worker. "It feels different. I'm very scared."

The Israeli military contends that Hezbollah is building its presence along the Blue Line in violation of international agreements. An opening in the white trailer, which is about the size of a shipping container, and windows in the tower peer directly into Israel.

Israel has asked the United Nations Interim Force in Lebanon (UNIFIL), the body responsible for monitoring the border, to intervene, but it claims its hands are tied.

A spokesman for UNIFIL said that it was aware of the 22 posts that have recently appeared, but that it was unable to visit because the Lebanese government has declared them private property. Nor had it detected any activities at the sites prohibited by the cease-fire agreements between the two countries.

"UNIFIL has not observed any unauthorized armed persons at the locations or found any basis to report a violation of [cease-fire agreements]," said spokesman Andrea Tenenti. "On our part, UNIFIL remains vigilant and continues to closely monitor all these sites and the Blue Line."

Israeli observers say they have watched men in the outpost taking photos of its equipment and personnel. The banner of an environmental group flies over the structures, "Green Without Borders," which Israel contends is a front organization.

"They may not hold up cards saying 'We are Hezbollah,' but these are not birdwatchers," said the senior official.

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The Washington Post's Sarah Dadouch in Beirut contributed to this report.