Egyptian aircraft struck at targets near the border with Israel on Wednesday and troops raided villages in a crackdown on Islamic militants blamed for a deadly attack on Egyptian border police, army officials and witnesses said.
The air strikes around the town of Sheikh Zuwaid, 10 kilometers (6 miles) from Palestinian-ruled Gaza Strip, followed clashes overnight between armed men and security forces at several checkpoints in the north of Egypt's Sinai region.
Gunmen killed 16 border guards on Sunday in the bloodiest attack on security forces in Sinai since Egypt signed a peace treaty with Israel in 1979, an accord which brought a close to a succession of wars between the two countries.
The attackers stormed through the border into Israel but they were killed by Israeli fire.
The militants, who are sworn to destroy Israel, have stepped up their actions along the isolated desert frontier since an uprising toppled Egypt's autocratic leader Hosni Mubarak last year.
The new president, Mohamed Mursi, a moderate who took office in June, has tried to allay Israeli concerns with promises to bring the region back under government control.
The Egyptian army, which kept peace with Israel throughout the Mubarak years, still keeps broad sway over national security. But Mursi has also brought Egypt closer to the Hamas movement ruling Gaza, making this a delicate time in relations between the Jewish state and the Arab power.
Israel said Egypt's action against the militants was a necessary response against groups threatening its security.
Egyptian army rockets have not been fired in the area since Egypt's 1973 war with Israel and the military operation appeared to be the biggest there since the peace treaty was signed.
The militant strongholds are in northern Sinai, away from the Red Sea resorts further south that are popular with foreign tourists and a lifeline for Egypt's struggling economy.
Witnesses in Sheikh Zuwaid said they saw two military planes fly over the area and heard explosions on Wednesday morning. Other people near the town said they saw three cars bombed.
Troops entered the village of al-Toumah, 20 kilometers (15 miles) further south, after the army received information that militants were staying there, military commanders in Sinai told Reuters.
”We have succeeded in entering al-Toumah village, killed 20 terrorists and destroyed three armored cars belonging to terrorists. Operations are still ongoing,” he said by telephone.
A villager said he saw military helicopters chasing vehicles out of al-Toumah and heard rocket fire. The men in the cars fired back with machine guns, he said.
An army general commandeering a unit in al-Toumah said the militants were hiding beneath a hill.
“They tried to escape in three vans but the helicopter met them on the other side, fired rockets at them and destroyed the three vehicles.”
After Sunday's attack, Israel had called on Egypt to end lawlessless near the border and the Egyptian army promised retribution, branding the attackers “infidels.”
But there was no crackdown until Tuesday night after armed men opened fire on several checkpoints in al-Arish town, the security and administrative centre for northern Sinai.
Gunmen also attacked checkpoints in Rafah, Egypt's entry point into the Gaza Strip that borders both Israel and Egypt.
Residents demand protection
Cairo's efforts to get a grip on Sinai are complicated by deep resentment towards the authorities that has made some local Bedouin unwilling to cooperate with the security forces to now.
The military response focused immediately on Shaikh Zuwaid, an economically deprived town that has come to rely heavily on profits from smuggling goods and people through tunnels into Gaza since the Palestinian territory was cut off from Israel.
The government in Cairo said the gunmen behind Sunday's attack had reached Egypt via the tunnels and it began work to seal them off on Tuesday.
Israel has long accused Palestinian jihadi groups of crossing from Gaza to Egypt to team up with local militants with the aim of attacking Israel's long border.
Eight Israelis have been killed in border attacks in the past year.
In al-Arish, a town relatively developed compared to others in the region, residents took to the streets overnight to demand better protection from the government and arms to defend themselves after the attacks on checkpoints.
Security forces closed al-Arish's main highway shortly after the start of the military operation. Power, Internet and mobile phone networks in the area were shut down.
“What we see in Egypt is a strong fury, a determination of the regime and the army to take care of it and impose order in Sinai because that is their responsibility,” senior Israeli defense official, Amos Gilad, said on Israel Radio on Wednesday.
Mubarak's government worked closely with Israel to secure the frontier region until he was toppled 18 months ago.
The revolt made way for Egypt's first free leadership vote which brought into office Mursi, whose commitment to security cooperation with Israel is now being tested.
Hamas condemned Sunday's attack and said it was trying to help Egypt identify the gunmen. But any proven Gaza connection could be uncomfortable for Mursi as he tries to assert his authority over a suspicious security establishment.