By Yousri Mohamed

ISMAILIA, May 20 (Reuters) - Gunmen stormed an Egyptian security forces base in the Sinai Peninsula at dawn on Monday and exchanged gunfire with forces inside the base before retreating, security sources said.

The gunmen attacked the base in the Al-Ahrash area in North Sinai from a truck and fired automatic weapons, but the attack did not result in any casualties.

The identity of the attackers was not immediately clear, but security sources said they were likely to be Islamist militants.

Hardline Islamist groups in North Sinai were emboldened by the overthrow of former President Hosni Mubarak in 2011. They have exploited the ensuing collapse of state authority and attacked targets in North Sinai and across the border in Israel.

Gunmen demanding the release of jailed Islamist militants seized seven members of security forces on Thursday.

The kidnapping has drawn national attention to lawlessness in the strategic peninsula and enraged security forces in the area, who have blocked border crossings into Israel and the Gaza Strip to pressure the government to help free their colleagues.

A video posted online on Sunday showed seven blindfolded men with their hands bound above their heads, who said they were the hostages, begging President Mohamed Mursi to free political detainees in Sinai in exchange for their own release.  The video, which was the first sign of the hostages since their kidnapping, could not be independently verified. State newspaper Al-Ahram said security services were looking into its authenticity.

Mursi said on Sunday that "all options are open" to free the hostages. "We will not succumb to any blackmail," he wrote on Twitter shortly after the video was posted online.

Presidential spokesman Omar Amer told Egyptian state television that no talks were taking place with the kidnappers and that it would be unacceptable to negotiate with criminals.

The army shifted several units of troops to North Sinai "in preparation for taking part in a large-scale military operation to release the abducted soldiers if negotiations came to failure," the government said in a State Information Service statement in English on Sunday. (Writing by Maggie Fick; Editing by Catherine Evans)