The Iranian-backed Lebanese Shiite movement Hezbollah is poised to launch a much-anticipated offensive to the north of Damascus in a counterinsurgency campaign that is likely to prompt hand-wringing in Washington and more Saudi frustration with Western inaction in Syria.

The battle for mountainous Al-Qalamoun-a rugged region between the Syrian capital and Homs, the country’s third largest city-will be as significant in military terms when it comes, say diplomats and analysts, as the struggle in the spring for Qusair, a strategic town in sight of Lebanon, that was retaken by the Syrian army thanks to Hezbollah, whose fighters were in the vanguard of the assault.

Qusair’s capture goaded President Obama in June to pledge he would arm the Syrian rebels-a promise that hasn’t been fulfilled because of the administration’s worries about the growing influence of al-Qaeda affiliates in the rebellion against President Bashar al-Assad.

The offensive will again pit Hezbollah fighters directly against jihadists and militant Islamists. The al-Qaeda affiliate Jabhat al-Nusra and Islamist militias Ahrar al-Sham and Liwa al-Islamhave been reinforcing towns and villages in the region to prepare for the expected Hezbollah assault. Some reports claim that as many as 20,000 rebel fighters have poured into the region, some being redeployed from Damascus suburbs.

A grueling confrontation in Al-Qalamoun-an area 50 miles long and 25 miles broad that runs from the rural outskirts of the Syrian capital to the Lebanese border-could see Saudi Arabia accelerate its arming of certain rebel groups that the Obama administration considers dangerous to the West, adding to strained relations between Washington and Riyadh.

Last week, Saudi Arabia spurned a long-coveted seat on the UN Security Council, in part to protest what it called the failure of the world body to punish Assad for alleged chemical-weapon attacks on civilians over the summer. But Saudi’s intelligence chief Prince Bandar bin Sultan stressed to European diplomats that the rejection of the UN seat was primarily meant as a message to the U.S. and an expression of anger at the Obama administration’s policies towards Syria, Egypt and Iran.

According to Reuters, Prince Bandar warned that American-Saudi relations are poised to get worse unless Washington’s foreign policy becomes more aggressive towards Syria and Iran. Increased Saudi arming ofjihadist and militant Islamist groups would certainly add to the tensions between the two allies.

The leader of Liwa al-Islam, Zahran Alloush, visited Saudi Arabia in September, holding talks with Prince Bandar. Rebel sources say the two discussed boosting Saudi assistance to the rebellion. Alloush is the son of a prominent Saudi cleric and was one of the driving forces behind the recent formation of a new Islamist coalition of rebel brigades that broke away from the Western-backed Free Syrian Army.

In a news conference earlier this month, the head of Lebanon’s pro-Assad Arab Democratic Party, warned that Saudi Arabia was planning to set Lebanon alight if Hezbollah joined the battle for Al-Qalamoun. “Saudi Arabia warned Hezbollah against participating in the battle,” he told reporters.

The Al-Qalamoun region is seen as vital both by Syrian forces and the rebels. Controlling Al-Qalamoun would allow the Assad regime to secure land links between Damascus and Homs and interdict arms supplies from Lebanese Sunni supporters for the rebels coming through the border around the town of Arsal.

For the regime, consolidating its hold on Homs is a priority, as it represents a central link between Syria’s interior cities and the Mediterranean coast north of Latakia, a stronghold of President Assad’s minority Alawite sect.

Hezbollah officials have been briefing Lebanese media outlets on an upcoming Al-Qalamoun offensive as part of an effort to manage and prepare their own followers, many of whom in the south of Lebanon have begun to express doubts about the wisdom of becoming further embroiled in their neighbor’s raging civil war.

In Shiite areas of the Bekaa Valley, backing for Hezbollah engagement in Syria remains high, partly because of close family ties between Shiites on both sides of the border.

But in the south, rare behind-the scenes disgruntlement is growing with the Hezbollah leadership, says Hisham Jaber, a Shiite and retired Lebanese army general. He says southern Shiite families are questioning the wisdom of Hezbollah fighting fellow Muslims, even if they are Sunnis.

“The family ties between the Shiite in the Bekaa and the Shiite in Syria is different than south Lebanon,” says Jaber. “People in south Lebanon don’t have such close ties with Syria.”

Lebanese officials and Western diplomats worry that Lebanon won’t be left unscathed in a prolonged battle for Al-Qalamoun. “This isn’t going to be a two-week battle like Qusair,” says a British military adviser to the Lebanese army. “The region is mountainous and the offensive will extend into the spring and there’ll be more chance of violent spillover into Lebanon.”

A Hezbollah fighter acknowledged in an interview with NOW, a Lebanese website, that the battle for Al-Qalamoun would be different from the fight over Qusair and would take much longer “because of the nature of the terrain, which is made up of high mountains and deep valleys.”

An offensive in the region was predicted some weeks ago, soon after the retaking of Qusair by pro-Assad forces. Many of the rebel fighters who escaped from that battle headed to villages in Al-Qalamoun. A Hezbollah special forces commander interviewed by The Daily Beast in the summer suggested an offensive would be launched quickly but it was instead delayed, possibly because of diplomatic fallout from the August chemical-weapons attacks.

FSA sources have warned of severe repercussions for Lebanon from a battle in Al-Qalamoun, involving a possible movement of rebel fighters into Lebanon and rebel rocket attacks on Hezbollah strongholds in the Bekaa Valley. In the summer, two car bomb attacks on a Hezbollah suburb of Beirut that left dozens dead frayed nerves.

Last Thursday, the Lebanese security forces intercepted a car carrying 250 kilograms of explosives and clashed with Syrian rebels in the Bekaa Valley.

Opposition activists in Yabrud, a village in the Al-Qalamoun region, say airstrikes and artillery bombardments have picked up pace in the past few days.