At least 16 Egyptian police officers have been killed and others wounded in an armed attack on a police station in north Sinai on the border between Egypt and Israel, authorities say.
The assault at Karem Abu Salem crossing took place on Sunday after gunmen reportedly tried to smash their way across the border into Israel.
Egyptian state television said that armed foreign fighters were behind the attack.
"State media is quoting unnamed officials saying that foreign fighters, armed groups, belonging to some previously unknown extremist group have infiltrated the country from Gaza through the tunnels criss-crossing the porous border area," Al Jazeera's Rawya Rageh reported from Cairo on Sunday.
"But we are unable to independently corroborate these allegations just yet," she said.
The attack comes a month after armed men believed to be Islamist fighters shot dead two Egyptian soldiers in a dawn raid in north Sinai.
Rageh said the border police patrol were ambushed by masked armed men while they were having their traditional meal at the end of the daily fast during the Muslim month of Ramadan.
Mohamed Morsi, Egypt's president, held an urgent meeting with the country's military, and promised a strong response to the attack.
"The president ... issued a statement in which he expressed condolences to the families who lost soldiers in this attack, but also issued an unprecedented stern warning in which he said that this attack will not go lightly and that the perpetrators will pay the price dearly," our correspondent said.
A senior security official, speaking on condition of anonymity because he was not allowed to speak to reporters, said seven other guards were wounded in the attack.
He said the attackers seized an armoured vehicle before driving away.
Israel pursues attackers
Israel said the attackers commandeered two Egyptian vehicles and tried to storm its border.
One of the vehicles exploded and the second was targeted by Israeli aircraft, Avital Leibovich, a military spokeswoman, said, adding that an unspecified number of the assailants were killed while trying to escape.
Ofir Gendelman, Israeli government spokesman, said seven attackers were killed, four on the Israeli side and three in Egypt.
She said Israeli soldiers were combing the area for other assailants who might still be on the Israeli side of the border. The military instructed Israeli civilians to stay inside their homes.
Ehud Barak, Israel's defence minister, said the attack showed need for "determined Egyptian action" to impose security and "prevent terror in Sinai".
In a statement, Hamas, the Palestinian group controlling the Gaza Strip near Sinai, condemned the attack, calling it an "ugly crime" and extended "deep condolences to the families of the victims and to the leadership and the people of Egypt".
The Sinai is home to Egypt's Red Sea resorts, a source of lucrative tourist income, and is also where the country's Bedouin, who were long marginalised under the regime of fallen president Hosni Mubarak, are based.
Before the July attack in Sheikh Zuwaid, a town roughly 15km west of the Gaza Strip, Islamist fighters had distributed pamphlets calling on the army, brought in to restore security, to leave the lawless north of the peninsula.
The military sent tanks and soldiers into the region last year to quell Islamist fighters, after receiving permission from Israel.
Under a 1979 peace treaty with Israel, Egypt should have a limited military presence in the area.