Egyptian military helicopters have fired missiles on suspected Islamist militants in Egypt’s northern Sinai peninsula, security officials say.Twenty people were reported killed in Touma village, while the Sheikh Zuwaid area to the west was also hit.The strikes came after security checkpoints were allegedly attacked by gunmen in the town of al-Arish, leaving a number of people injured.On Sunday, militants killed 16 Egyptian border guards in the area.After that attack - the deadliest and most brazen against Egyptian troops in this border region for decades - Israeli forces said they killed some of the militants who broke through into Israel.There has been a heavy military build-up around al-Arish, correspondents report, and Egypt’s Rafah border crossing to Gaza has been indefinitely closed as security forces hunt the remaining attackers. Egypt is also reported to have begun sealing off the illicit smuggling tunnels into Gaza. Operations ‘ongoing’ This is the first time Egypt has fired missiles in Sinai since the 1973 war with Israel, when it attempted to recapture the Sinai peninsula, security officials told Associated Press.Egyptian military presence in Sinai is limited and requires Israeli approval under the terms of the 1979 peace treaty between the nations which returned Sinai to Egyptian control.Tensions are very high in the area, where Islamic extremists are said to have gained a foothold in recent months, taking advantage of the security vacuum left after former President Hosni Mubarak was ousted last year, reports the BBC’s Yolande Knell in Sinai.A Sinai army commander told Reuters news agency the army had received information that there were many militants in Touma."We have succeeded in entering Touma, killed 20 terrorists and destroyed three armoured cars belonging to terrorists. Operations are still ongoing,” he told Reuters. State news agency Mena said "intensive security campaigns” had been launched against "terrorist hotbeds” in southern al-Arish, Sheikh Zuwaid and Rafah.It said that during the air raids "terrorists” had used rockets and rocket-propelled grenades to try to shoot down the military aircraft "but they failed”.The air raids came hours after security checkpoints were attacked in the main regional town of al-Arish, sparking clashes with security forces.Locals said rounds of gunfire could be heard just before midnight and telephone lines and the Internet were cut off.