EL-ARISH, Egypt (AP) — Egyptian troops and security forces on Friday detained nine Islamic militants in northern Sinai believed to be behind a surprise attack last weekend that killed 16 Egyptian soldiers, a security official said.

It was the first reported arrest in connection with the attack, which took place last Sunday and which sparked a major Egyptian military operation in the Sinai Peninsula aimed at stamping out Islamic militant groups that have become bolder and grown in numbers since the ouster last year of Hosni Mubarak.

So far the effectiveness of the 4-day-old operation is not clear. Despite the influx of troops, militants have continued low-level attacks on Egyptian troops and security forces. One famous checkpoint on the road linking Rafah border town with the city of el-Arish comes under attack almost daily. Officials say that militants open fire at night, engage in brief firefight then flee.

In the raid early Friday morning, troops stormed a house in Sheik Zweid, close to the Rafah border crossing with the Gaza Strip, and caught the nine suspected militants while they were asleep. Among them was Selmi Zeyoud, whom the official described as a “dangerous element” and a brother to a slain jihadist. The official spoke on condition of anonymity because he was not authorized to speak to the press.

The official said the nine were suspected of involvement in the stunning assault last Sunday, in which gunmen stormed an army checkpoint by the borders with Gaza and Israel, and killed 16 soldiers as they were breaking their daily fast for the holy month of Ramadan with a sunset meal. The attackers then commandeered an armored vehicle, which they later used to storm across the border into Israel where they were hit by an Israeli airstrike that killed at least six militants.

Large swaths of northern Sinai have plunged into lawlessness following the ouster of Mubarak in last year’s uprising, and weapons smuggled from Libya have found their way into militants’ hands. The weapons and the security vacuum fueled the rise of al-Qaida-inspired militant groups which have staged several low-level cross-border attacks on Israel.

Security officials estimate the number of militants in northern Sinai to some 1,500 but some Bebouin tribal leaders have put the number in the thousands. One prominent tribal chief, Awda Abu-Malhous, put it as high as 10,000.

The military sent tanks and troops to the peninsula to combat the terror groups. However, witnesses say the offensive has been limited to a few raids on houses of suspected militants. The Al-Ahram newspaper and other state-run papers reported Friday that 60 “terrorists” have been killed in airstrikes. However, medical officials say no bodies reached el-Arish’s only hospital.

Officials said earlier that along with the offensive, Egypt is going after an elaborate network of underground tunnels used to smuggle weapons, militants and goods between Sinai and Gaza. Al-Ahram reported that 150 tunnels have been destroyed. Resident in the area said the tunnels targeted were not the most active ones.

The local Bedouin population is largely resentful of the central government over years of discrimination, marginalization and heavy-handed security sweeps under Mubarak. Some have nonetheless helped the military, though many are fearful of risking their lives if they help security forces and provide tips on the militants — many of whom are Bedouins — after one of tribal leader was shot dead two months ago for cooperating with the police.

The weekend attack has rattled the Egyptian government, the military council and the country’s Islamist president Mohammed Morsi, who fired the chief of intelligence in an apparent attempt to defuse public discontent.

Copyright 2012 The Associated Press