Five Egyptian police officers were gunned down in separate incidents on Friday in the North Sinai town of El Arish, medical sources said, after Islamist gunmen killed a soldier in a separate attack in a nearby town overnight.

The police officers were shot by gunmen while they were guarding a government building, a checkpoint on the southern outskirts of the town, and the hospital, the sources said.

It was not clear whether the attacks were coordinated and in reaction to the army's overthrow of elected Islamist President Mohammed Morsi on Wednesday.

Troops and Morsi supporters also clashed at the other end of the peninsula, in the cities of Suez and Ismailia on the Suez Canal, witnesses and security sources said.

Egypt has struggled to control security in the Sinai peninsula since the ousting of autocratic President Hosni Mubarak in 2011. Hardline Islamist groups have exploited the collapse of state authority after the uprising to launch attacks into Israel and on Egyptian targets.

Troops were "on alert" in the peninsula, a military spokesman said, but he denied a report by the state-owned Al-Ahram newspaper that a state of emergency had been declared in Suez and South Sinai provinces.

"We were already very prepared in this important strategic region," a military source said. "The announcement is just to assure our soldiers and people that we are ready in Sinai. As for Suez we are always in an alert state there."

The head of the Suez Canal Authority, who oversees the key world trade route, said it was operating normally with no disruption to cargo traffic. Mohab Mameesh said 48 ships had passed through the canal on Friday.

Security sources said a soldier was killed and two were wounded when a police station in Rafah on the border with the Gaza Strip came under rocket fire. The police post is close to the local headquarters of military intelligence.

Earlier, attackers fired rocket-propelled grenades at army checkpoints guarding El Arish airport.

State television reported later Friday that Egyptian authorities imposed a curfew in two North Sinai towns on the border with Israel and the Gaza Strip. The curfew applies to the towns of Sheikh Zuweid and Rafah, it said, without elaborating. The army spokesman later denied the curfews were imposed. 

Islamist militants believed to have links to Al-Qaida have established a foothold in the sparsely populated desert peninsula, sometimes in league with local Bedouin smugglers and with Palestinian militants from Gaza.