Israel and the Palestinian Authority admitted they fear al-Qaida terrorists may infiltrate into Gaza through the open Gaza-Egypt border, where Palestinians and Egyptians have been crossing freely since Israel withdrew from the area.
In a deal worked out with Israel, Egypt is supposed to deploy 750 border troops to secure the frontier and prevent weapons smuggling, but neither those troops nor Palestinian policemen have been able to halt the flow of people and arms, including hundreds of assault rifles and pistols.
Israel fears international terrorists will exploit the chaotic border to infiltrate Gaza and Israel.
"We're talking about Iran, we're talking elements in Syria, we're talking about groups like Hezbollah and we're talking also about international terrorist groups like al-Qaida," said Israeli foreign ministry spokesman Mark Regev. Israel has long accused both Iran and Syria of sponsoring militant groups.
As it it well known islamic militant groups, some claiming connections with al Qaida, have been active in northern Egypt.
Senior Israeli military officials said they feared al-Qaida operatives could enter Gaza from Sinai and connect with the local Hamas militant group to share expertise and provide weapons.
Israel's border patrol arrested 21 Gazans trying to get into Israel through Sinai overnight, said border patrol spokesman Ilan Azini. He said the infiltrators were apparently either trying to enter Israel to get work or attempting to reach the West Bank.
There were some signs Thursday that security forces were beginning to stem the free flow of people across the border, with Egyptian troops allowing Palestinians to re-enter Gaza but often preventing them from crossing into Egypt. At one point, Palestinian police fired into the air to prevent people from crossing into Egypt.
However, in other places Palestinian police and Egyptian troops stood by idly as people crossed over, the AP reports.
The strike was defensive in nature and came in response to three attacks on the US military in February