CAIRO (Reuters) - Egypt began allowing Palestinians wounded in an Israeli offensive in the Gaza Strip to cross into Egypt for medical care on Sunday via Gaza's closed southern border, Egyptian security sources said.

Israeli forces killed 61 people, nearly half of them civilians, in the Hamas-controlled Gaza Strip on Saturday, the bloodiest single day for Palestinians since the 1980s. Two Israeli soldiers were also killed fighting Gaza militants.

Egyptian security sources at the Rafah border crossing said four wounded Palestinian civilians crossed into Egypt on Sunday. Twenty ambulances waited at the border to take more wounded.

The sources said Egypt had not set a limit on how many Palestinians would be allowed in, but was expecting at least 100. Hospitals in the northern Sinai were on alert and state news agency MENA said some wounded could be taken to Cairo for treatment.

Israel vowed on Sunday to press on with its Gaza offensive and curb rocket strikes, threatening stronger action despite U.N. condemnation of assaults that have killed more than 100 Palestinians in five days of fighting.

Israeli Prime Minister Ehud Olmert has been under pressure from some cabinet members to launch a broader offensive in the Gaza Strip, especially after militants began firing longer-range rockets at Ashkelon, a city of 120,000 people.

The Gaza-Egypt frontier at Rafah was resealed last month after Hamas militants blew it open in January in defiance of an Israeli-led blockade, a move that prompted Palestinians to flood into Egypt to stock up on supplies.