CAIRO — Islamist militants shot down an Egyptian military helicopter in the Sinai Peninsula with a surface-to-air missile over the weekend, raising new alarms about the terrorist insurgency that developed there in response to the military takeover last summer.

The attack — described by witnesses, documented in a video released by the militants, and confirmed by three people briefed on the Egyptian government’s investigation — validated longstanding fears that such weapons would spill into Egypt and beyond after the Libyan civil war tore open Col. Muammar el-Qaddafi’s fearsome arsenals.

Known as manpads, for man-portable air defense systems, the missiles can bring down commercial airliners if they are flying at low altitude, as during takeoff and landing. They also strengthen Sinai-based militants’ hand against the Egyptian Army, and there is concern that the militants might seek to use the missiles on the Egyptian mainland, where terrorist attacks have become more frequent. On Saturday, four bombings killed six people in greater Cairo.

Ansar Beit al-Maqdis, a militant group based in Sinai that has claimed responsibility for an escalating series of attacks on police and soldiers, said in a statement posted on jihadist websites that it had succeeded in “downing a military helicopter with a surface-to-air missile and killing its entire crew in the area around the city of Sheikh Zuweid” in North Sinai, near the border with Gaza. The group, which quotes Al Qaeda leaders in its online video statements, said it had targeted the aircraft “in an area clear of residents to preserve the lives of Muslims.”