BEIRUT — Syrian opposition fighters blew up bridges Friday and dug trenches around their bases to impede an anticipated ground offensive on their last major stronghold in the country. They also called on residents to take up arms and support front-line fighters.

The looming battle for Idlib in northwestern Syria may be the last in the bloody seven years of conflict, which have backed hundreds of thousands of civilians into this deadly corner of the country with nowhere to run.

"This is our last chance to be free. The uprising is about to end," said Abdulkafi Alhamdo, a 33-year old English teacher, who is awaiting the imminent birth of his second child.

Idlib and the surrounding area is home to some 3 million people — nearly half of them, including Alhamdo, already displaced more than once by the civil war — choosing to live in opposition areas.

On Friday, U.S. Secretary of State Mike Pompeo said 3 million Syrians "will suffer" from this aggression.

"The U.S. sees this (looming Russian-backed Syrian assault) as an escalation of an already dangerous conflict," Pompeo tweeted.

U.N. officials believe an offensive on Idlib would trigger a wave of displacement that could uproot up an estimated 800,000 people and discourage refugees from returning home as they see a new wave of violence unfold.

U.N. High Commissioner for Refugees Filippo Grandi, speaking in Lebanon after a visit to Damascus, said he had appealed to both sides in the conflict "to try to find a solution, a way forward" that spares civilians.

"If you have violent military action and loss of lives, you risk of course many deaths ... a human catastrophe," he told reporters. "But you risk also sending a message to refugees that the situation is not secured. (They) will be watching very carefully what is happening in Idlib in the next few months."

Thousands of government troops and allied fighters have been amassing in areas surrounding Idlib. Russia has said a military operation there is necessary to weed out "terrorists" it blames for attacking its bases on the coast.

The Britain-based Syrian Observatory for Human Rights said Syrian forces have deployed at least 2,000 armored vehicles along the front lines surrounding Idlib and Hama.

An offensive is likely to first strike southwest Idlib and al-Ghab plains, which overlook the coastal area where Russia has its military and naval bases. Another front for the offensive is from the south and southeast, which would restore government control over an essential highway that runs between Syria's major cities.

The area, controlled by rebels since 2015, has been targeted by government and Russia strikes for months, leaving its infrastructure, schools and hospitals in desperate conditions. Now a wide-scale offensive is in the offing, with Damascus and its allies intent on making it a defining moment in the seven-year conflict.

Cornered between a tightly sealed border with Turkey and government areas, residents of the rebels' last bastion also live among the most battle-hardened fighters from all over Syria and thousands of foreign fighters who make up a significant part of Hayat Tahrir al-Sham (or Levant Liberation Committee), an al-Qaida-linked group that controls most of the territory. For those fighters, the battle for Idlib will likely be the last showdown with the government and they are expected to put up a hard fight.

The rebels' controlled demolitions of the bridges came after they detected movement of government troops in the area, according to Rami Abdurrahman, head of the Observatory.

Most of Idlib province and adjacent strips of Hama province remain in the hands of an assortment of armed groups, some Turkey-backed and others independent Islamist groups. But the strongest alliance of fighters is led by the LLC that controls most of the area.

The group has called on residents' support by way of fortifying the province's perimeter, taking up arms alongside their fighters or volunteering at front-line hospitals and kitchens.

On Friday, thousands protested in various Idlib and Hama towns, denouncing threats of an attack and trumpeting the area's readiness to fight. The LLC's administrators had called for the protests.

Unlike other areas that have been reclaimed by recent government offensives, Idlib is not totally besieged. Its borders with Turkey and adjacent territories administered by Turkish troops are open, allowing in fuel, produce and other products. But trade with government-controlled areas has been stunted as rebel-operated crossings have been blocked by the government since mid-August.

Some residents say they are feeling the pinch.

Bassam Abu Bashir, an anesthesiologist, said fuel is already in short supply in Maaret al-Numan where he lives after having been displaced from eastern Ghouta near Damascus earlier this year following a military campaign.

Bracing for a new onslaught, he said he is looking to relocate his 12-member extended family to Afrin, a nearby town administered by Turkish authorities and its Syrian allies. But transportation and rent are costly. "The rent is nearly a $100 and I need to pay six months in advance. Otherwise, camps for displaced people are our only option," he said.

The road to Afrin is also not secure, as Turkish troops and allied forces battle Kurdish fighters that have been uprooted from the area and are threatening to return.

A separate, equally unnerving, battle is also unfolding across media.

Faysal al-Antar, a member of Kfar Zita local council in northern Hama, said the town on the front line with government troops has been under constant shelling for years.

Yet in recent days, Russian officials and Syrian media have accused activists of preparing to launch a chemical attack, with Kfar Zita as its staging ground, and blame Damascus, prompting western retaliation. France and U.S. have said a chemical attack won't be tolerated while Damascus denies it has such weapons.

Al-Antar dismisses the allegations as a ploy to scare and drive out locals, enabling troop advances on his hometown. Nearly 1,500 families remain in the town, he said, but there are no operating medical centers or schools. The town's only bakery was damaged in previous strikes.

"There is fear and anticipation among residents," he said, saying most of those who stayed in Kfar Zita don't have the means to travel or leave. "We have no equipment to face such a chemical attack if it occurs and we have nothing to fight it with."

In Idlib city, Alhamdo, the father-to-be, said all he can do is wait. He has refrained from making any purchases for his wife or the child they're expecting in order to protect his savings.

Having lived through a government assault on his hometown in Aleppo, Alhamdo said he is probably more concerned than others because he knows what lies ahead.

"We are between two sides ... the end of the world that is the border with Turkey and the enemy, or hell, which is coming to burn everything," he said in a series of voice messages sent to The Associated Press.

Turkey says it is seeking to ward off a full-scale offensive, urging Russia and Iran to allow more time to separate "radical militants" from the armed groups it has backed.

"We are working toward stopping this attack," Turkish Foreign Minister Mevlut Cavusoglu said in Vienna Friday.

Turkey fears a humanitarian crisis on its borders. The country's defense minister and intelligence chief have visited Moscow while Iran's foreign minister has held meetings in Ankara over the past few days. A summit is expected in Iran between Iranian, Turkish and Russian leaders next week.