The timing of Moshe Yaalon's comments also appeared unusual, given that Egypt spent the previous day brokering an end to the latest round of fighting between Israel and Palestinian militants in the Hamas-run Gaza Strip after several days of rocket fire from Gaza and Israeli airstrikes.
Militants operating in the lawless Sinai, which borders both Israel and Gaza, have launched attacks on Israel and smuggled weapons through tunnels into Gaza for use against Israel. The Islamist bands have also targeted Egyptian forces.
Asked whether Egypt was doing enough against weapons smuggling through Sinai, Yaalon told Israel Radio that Israel cooperates on security matters with Egypt's new government headed by President Mohammed Morsi of the Muslim Brotherhood, the Hamas parent movement.
Still, Yaalon added, "To say that we are satisfied with Egyptian activity in Sinai, I can't say we are satisfied yet."
"Sinai has become lawless territory," he said. "It's a question of Egypt deciding to assert its sovereignty the way it should and acting resolutely against terrorists. I hope that's what will happen. It hasn't happened yet."
Government spokesman Mark Regev had no comment when asked whether Yaalon was articulating his personal opinion or an official government position.
Israel's ties with Egypt, never warm, have been strained since the ouster of longtime President Hosni Mubarak in a popular uprising in February 2011.
Israel is worried that Egypt's new Islamist government might not preserve the two countries' landmark 1979 peace treaty, and security in Sinai has deteriorated, with a sharp rise in militant attacks on targets both in Egypt and across the border in Israel.
For all that, Israeli officials have been careful to keep their criticism of Egypt quiet since the formation of Morsi's government. The peace agreement with Egypt is critical to Israeli security, and Israeli leaders do not want to damage the already strained relationship.
The Brotherhood has reaffirmed its commitment to the treaty, while calling for changes in the limits on troop and equipment deployments in Sinai that the 1979 agreement imposed at Israel's insistence.
Since the Mubarak government fell, the Egyptians have deviated from those limits in efforts to crack down on militants, obtaining quiet Israeli permission in all but one case.
Yaalon told Israel Radio that Israel would not formalize changes in the treaty.