Lebanese soldiers raided an apartment in the country’s south on Saturday and arrested five Syrian nationals for possession of explosives, the army said, the latest incident fanning fears that Syria’s civil war is spilling across the border.

The arrests came on the eve of Ashoura, the annual Shiite commemoration of the 7th-century death of Imam Hussein, the Prophet Muhammad’s grandson. Shiite processions — one of which will take place in Nabatiyeh where the arrests occurred — have been attacked by Sunni extremists in Iraq and other countries.

According to Lebanese media reports, the five suspects, four women and one man, intended to strike Shiite mourners loyal to Hezbollah observing Ashoura, because of the terror organization’s support of Syrian President Bashar Assad. An army spokesman declined comment to The Associated Press, pending judicial investigations.

Lebanon and Syria share a complex web of political and sectarian ties and rivalries which are easily enflamed. Lebanon, a country plagued by decades of strife, has been on edge since the uprising in Syria against President Bashar Assad began in March 2011, with deadly clashes between pro and anti-Assad Lebanese groups erupting on several occasions.

Many among Lebanon’s Sunni Muslims have backed Syria’s mainly Sunni rebels, in which radical Islamists have become increasingly active. Lebanese Shiite Muslims and the militant group Hezbollah have tended to back Assad, whose tiny Alawite sect is an offshoot of Shiite Islam.

More than 40,000 people have been killed over the past 20 months in Syria since the start of the civil war against Assad, according to opposition groups.

In this latest incident, an army statement said the five Syrians were arrested in the southern market town of Nabatiyeh following a tip that they were involved in “suspicious security activity.” Army personnel seized 450 grams (one pound) of explosives, a detonator, and ammunition for a 160 mm mortar with Hebrew writing on it, the statement said.

Last month, Lebanon plunged into several days of violence after the country’s intelligence chief, Brig. Gen. Wissam al-Hassan was assassinated in a Beirut car bomb.

Al-Hassan was a powerful opponent of Syria’s influence in Lebanon and many here blamed his killing on Syria.