The battle lines in Lebanon are drawn sharper and sharper. They are largely sectarian and tied into the bitter civil war in neighbouring Syria.

On the one side are Sunni Muslim groups, especially Mustaqbal (Future), allied to Saudi Arabia and sympathetic to Syria’s Sunni rebels and the Sunni jihadis from around the region fighting on their side. On the other are the Shia Muslim parties Hezbollah and Amal, allied to Iran and to Syrian president Bashar al-Assad, whose regime draws heavily from the minority Alawite community that has origins in Shiism.

Detente between the United States and Iran has had no impact. “It cannot easily stop the momentum of what has already started,” says Yasser Akkaoui, editor-in-chief of the Beirut business monthly Executive.

Mustaqbal recently took huge advertising billboards featuring the Saudi King Abdullah gazing down benignly alongside the strap “The King of Good Deeds, the Kingdom of Giving”, in thanks for his $3 billion (£1.8 billion) gift to reequip the Lebanese army with French weapons.

How much of the money will disappear due to Lebanon’s rampant corruption is as unknown as the real purpose of the grant, variously speculated as the latest spasm of the Saudis’ huff against the US over its talks with Iran, a means for the Lebanese army to contain Hezbollah or a means for the army to contain violent Sunni groups.

The growing strength of militant Sunnis – generally called Salafis because they want to return to the “purity” of Islam’s early days – contrasts with their former lack of headway in Lebanon’s cosmopolitan and free-wheeling environment. Some profess an affinity with al-Qaeda, including Al-Nusra Front, which has claimed two bombings in January in mainly Shia southern Beirut, the most recent on 21 January killing at least four, including a teenage girl.

Many Lebanese are bewildered. They talk of audaa (the situation) and shrug their shoulders. Older people fight back memories of the 1975–90 civil war. They again feel caught in a crossfire – so far mainly of words, but ones that convey an increasing threat.

After November’s double suicide bombing at the Iranian embassy killed 23 people, the Hezbollah leader Hassan Nasrallah accused Saudi Arabia. When in December a car bomb in downtown Beirut killed Mohammad Chatah, a former Sunni finance minister, and six others, Mustaqbal leader Saad Hariri immediately blamed Hezbollah.

Then when Majid al-Majid, the Saudi leader of the militant Sunni group reportedly responsible for the embassy bombing, died in Lebanese army custody in early January, it was widely rumoured – and reported in some Iranian media outlets – that he had been killed by the Saudis to bury his knowledge of Salafi operations in both Lebanon and Syria.

Each side blames the other for the growth of militant Salafi groups, whose violence has also included December’s bombing of Lebanese army checkpoints in the mainly Sunni city of Sidon. Mustaqbal and its allies argue Hezbollah has brought the Sunni militants into Lebanon through fighting for President Assad in Syria. Hezbollah says it is in Syria resisting rebel Sunni groups to stop their sectarian violence from reaching Lebanon.

Iran’s critics do not see US-Iran detente as an opportunity to ease Shia-Sunni tensions. Rather, they are more than ever convinced it is trying to extend its regional sway by bolstering both Assad and Hezbollah.

In a speech in December, Mustaqbal parliamentary deputy Nuhad Mashnouq spoke of “resisting ... an Iranian revolutionary occupation of Lebanon’s decision-making”. Mashnouq rejected Nasrallah’s claim of Saudi involvement in the Iranian embassy bombing and argued the Saudis’ work for Muslim and Arab unity belied Iran’s “aggression against the Arabs”.

Yezid Sayigh, senior associate of the Carnegie Middle East Center in Beirut, traces the regional rise of Sunni-Shia sectarianism to the 2003 US invasion of Iraq, which prompted Sunnis from Lebanon, Syria, Jordan and the Palestinian refugee camps to go to fight the Americans and later against what they saw as a Shia-dominated Iraqi government.

But Sayigh also sees sectarianism as a deliberate ruse of Sunni regimes – Saudi Arabia, Mubarak’s Egypt, and King Abdullah of Jordan – to resist Iranian influence. “This was geopolitics, confronting Iran and opposing shifts in the balance of power they saw as hostile. By turning this into a sectarian conflict they were trying to countermobilize. We saw this in 2006 when Hezbollah went to war with Israel and the same countries initially responded by either staying quiet or blaming Hezbollah. They were uncomfortable with Hezbollah’s popularity crossing the sectarian divide.”

For Sayigh, Saudis’ volte face against Assad was rooted in its concern over Iran. “In January 2012, the Saudis were still backing Arab League mediation [to reconcile Assad and the rebels]. Then suddenly they shifted tack completely, and took the issue to the UN security council in a great rush, seeking a resolution to oust Bashar al-Assad.”

The timing, Sayigh believes, relates to Iran. “This was when the Obama administration was giving real signs it was not going to militarily strike Iran’s nuclear programme. We now know the Obama administration was in secret talks with the Iranians for the past year, prior to the preliminary [Geneva nuclear] agreement. January 2012 was ahead of that, but the Saudis must have known the Americans had more or less made up their minds to try and find a diplomatic way. So they thought, ‘If the Americans are not going to clobber Iran militarily, then we need to do what we can to clobber the Iranians elsewhere.’”

This suggests that for the Saudis, Syria is almost a sideshow. But by contrast some Lebanese critics of Iran argue that calculations over Lebanon and Syria are central to Tehran, and indeed are more important than the nuclear programme. Michael Young, opinion editor of the Beirut Daily Star, said “conservative elements” in Iran might well seek greater regional influence as compensation for accepting the conciliatory approach of President Hassan Rouhani over nuclear issues. “I would assume we will see more continuity in the coming months than many expect, especially as the Syrian situation is turning to Assad’s advantage, showing that the strategy of the more hardline elements in Iran was sound,” he said.

The intractable nature of the Syrian conflict embitters Lebanon’s own political paralysis. Since the cabinet resigned in March, a caretaker government has limped along, unable to take basic decisions or pass legislation as the leading parties bicker over a formula for a stable government.

Ironically, nine years after the Cedar Revolution in Lebanon led to the withdrawal of Syrian troops there are more Syrians than ever in the country. And no firm security structure has replaced the Syrian one. An employer in a sensitive area compared two similar incidents before and after Syrian withdrawal.

“In 2001, a man using a white car was photographing my staff coming and going. I knew who to call and sort out the problem,” he said. “In 2010, when something similar happened I called the army and the intelligence services and no one knew whose the car was.”

Unable two years ago to agree a strategy for dealing with Syrian refugees when the situation was still manageable – Hezbollah and other Assad allies opposed building camps in fear they would become rebel bases – the authorities now find themselves unable to count, much less monitor, around 1.5 million Syrian visitors who are badly straining already decrepit systems of electricity supply, health care and transport.

There is no public discussion in Lebanon of the long-term social or demographic consequence of this 25% population increase. Yet Syrians working informally have driven down wages, adding to the 20% of Lebanese already at or below a poverty line of $4 a day. And poorer Syrians, poisoned by sectarian conflict at home and facing discrimination in Lebanon, are fertile Salafi recruiting ground.

No one doubts it can get worse. With the government failing to curb its deficit, public debt has reached 138% of GDP, one of the highest ratios in the world. The Lebanese banks, which hold 58.5% of the debt, are nervous, aware the situation is tenable only as long as their big depositors – many expatriate Lebanese or Gulf Arabs – believe the risks of keeping money in Lebanon are outweighed by high interest rates and the banks’ past reliability.

Talks between Iran and the world powers aim at going beyond the interim, six-month agreement over Tehran’s nuclear programme reached in Geneva in November. But the diplomacy and handshakes are far away. Few in Lebanon expect even a substantial deal between the US and Iran will help “the situation” here.