GAZA (Reuters) - An Egyptian military helicopter briefly crossed into Israeli-controlled airspace over the Gaza Strip on Friday, in a possible sign of increased security jitters a week after Egypt's army toppled President Mohamed Mursi.
Security sources in Egypt and Israel both described the flyover as a navigational error, but it came shortly after militants killed an Egyptian policeman and wounded a second in an attack on checkpoints in the lawless Sinai Peninsula across the border from the Palestinian Gaza Strip.
Separately, Egyptian authorities arrested three Palestinian gunmen on Friday during "an attempt to attack vital sites in Sinai", Egyptian state media reported.
"The helicopter mistakenly crossed into Gazan airspace and immediately returned to Egypt," the Israeli security source said. Witnesses in Gaza said it stayed on their side of the border for about 10 minutes before returning.
The militant Palestinian group Hamas, which governs the Gaza Strip, declined comment on the helicopter incident. Hamas denies Egyptian accusations that it has an armed presence in Sinai.
Militants have staged almost daily attacks on security checkpoints in Sinai since the army's overthrow of the Islamist Mursi, Egypt's first democratically elected leader. Officials say Mursi is still being held at a military compound in Cairo.
Egyptian military helicopters were also seen dropping flyers on a pro-Mursi rally in Al-Arish town in northern Sinai urging them to denounce violence and to not drive cars without valid number plates, witnesses said.
Tens of thousands of pro-Mursi supporters also staged protests in different parts in Cairo on Friday.
Egyptian army sources have said an offensive in the Sinai against hardline Islamist militants may be imminent, though the military is now heavily focused on maintaining order in the streets of Cairo, where its tanks are deployed.
Under a 1979 Egyptian-Israeli peace deal, Sinai is largely demilitarized. But Egypt, with Israel's consent, has at times sent in extra forces to make security sweeps.
(Reporting by Nidal al-Mughrabi in Gaza, Ari Rabinovitch in Jerusalem, Yusri Mohamed, Noah Browning and Yasmine Saleh in Cairo; Editing by Michael Roddy)