Security has recently been stepped up outside the country's 12 Palestinian refugee camps, which are home to about 400,000 predominantly Sunni refugees.
But the camps are no-go areas for the Lebanese army and their internal security is handled by Palestinian political factions, with militant Sunni groups exploiting Lebanon's continuing political deadlock to establish themselves within the camps.
In Nahr al-Bared, a 30,000-strong refugee camp outside Tripoli in northern Lebanon, the Palestinian-born fugitive Shakir al-Abssi is developing his little-known group Fatah al-Islam, while amassing an arsenal that includes explosives and rockets. Recent independent polls show that half of Lebanon's population own, or have easy access to, weapons.
"It only takes a gun to shoot someone and cause a massive problem," said Timur Goksel, who was for two decades a senior UN adviser in Lebanon.
Stridently disowned by the Palestinian Fatah party of which it purports to be an offshoot, Fatah al-Islam raised concerns among Lebanese security officials late last year, after reports that 200 of its members had infiltrated the Palestinian refugee camps of Badawi and Nahr al-Bared, in northern Lebanon.
Amid Lebanon's worst political crisis since the 1975-90 civil war, there is a growing gulf between the Shia-led opposition movement, spearheaded by Iranian-backed Hezbollah, and the western-backed, Sunni-led government.
Sunni groups are reported to be funded by individual contributions from oil-rich Saudis, seeking to offset the influence of Hezbollah, the militant Shia group whose popularity in southern Lebanon was bolstered by its perceived victory in last summer's war against Israel.
Jawad Adra, the managing partner of the Beirut-based independent research body Information International, said that the increasing polarisation was providing a fertile breeding ground for anti-Western extremism among Sunni communities around Tripoli and Sidon.
"Western countries need to be careful that their political support for Lebanon's Sunni leaders, to offset the supposed Shia threat posed by Iran through Hezbollah, does not play into the hands of Sunni extremists," he said.
"The growth of Sunni extremism, not just in the Palestinian camps but also in impoverished areas of Lebanon, is a ticking time-bomb that is waiting to explode, and could sweep all moderates out of its path."