QAA, Lebanon — Every day around sunset, dozens of residents of this small Lebanese Christian village on the border carry their automatic rifles and deploy on surrounding hills, taking up positions and laying ambushes in case Muslim extremists from neighboring Syria attack.

“We all know that if they come, they will slit our throats for no reason,” said one villager as he drove through the streets of Qaa, an assault rifle resting next to him.

For months, Lebanese Christians have watched with dread as other Christians flee Islamic extremists in Syria and Iraq, fearing their turn will come next. Fears multiplied after militants from Syria overran a border town last month, clashing with security forces for days and killing and kidnapping Lebanese soldiers and policemen.

Now, for the first time since the Lebanese civil war ended in 1990, Lebanese Christians are rearming and setting up self-defense units to protect themselves, an indication of the growing anxiety over the expanding reach of radical Islamic groups.

Across the Middle East, Christian communities as old as the religion itself feel their very survival is now at stake, threatened by militants of the Islamic State group rampaging across Iraq and Syria.

In Iraq, thousands of Christians have fled their homes after they were made to choose between leaving, converting to Islam or facing death. For the first time in centuries, Iraq’s Ninevah region and the provincial capital of Mosul have been emptied of Christians. After they left, the militants spray-painted their houses with the letter “N’’ for “Nasrani” — an archaic term used to refer to Christians — marking the homes as Islamic State property.

In Syria, thousands of Christians have been displaced during its three-year conflict. Christian towns and villages have come under attack by jihadists, most recently the historic central town of Mahradeh. Islamic fighters in Syria rampaged through the ancient Christian town of Maaloula near Damascus earlier this year, destroying historic churches and icons. Christians in the militant stronghold of Raqqa were forced to pay an Islamic tax for protection.

Christian refugees from Iraq and Syria are now sheltering in Lebanon, sensing safety in a pluralistic country which has the largest percentage of Christians in the Middle East. Lebanon is also the only Arab country with a Christian head of state.

But the fear has spread to Lebanon as well. This week, after a video was posted online showing a group of boys burning an Islamic State flag in a Christian neighborhood of Beirut, vandals spray-painted the outer walls of several churches in northern Lebanon with the words: “The Islamic State is coming.”

In Qaa and Ras Baalbek, two Christian villages in the northeast, on the border with Syria, the anxiety is palpable. Many of the thousands of expatriates who used to spend the summer here stayed away this year. Restaurants and the villages’ main squares were deserted on a recent day.

The sale of weapons on the black market has climbed sharply. The arming effort is backed by some leftist and communist Lebanese militias who have long had weapons. The Shiite armed group Hezbollah has also indirectly supported such efforts, seeing the communities as a first line of defense for Shiite towns and villages in Lebanon’s eastern Bekaa region.