Prime Minister Ehud Olmert warned Palestinian militants on Tuesday that Israel would respond with force if attacks from the Gaza Strip persist in defiance of a recent truce.

Earlier in the day, Israel once again closed its cargo crossings, accusing Palestinian militants of firing a rocket at southern Israel in violation of a shaky truce.

Gaza's Hamas rulers denied a rocket was fired on Monday evening, and accused Israel of looking for excuses to shirk its commitment under the cease-fire agreement to ease its blockade of impoverished Gaza.

Militants have attacked Israel at least three times since the June 19 truce. Israel has responded by closing its cargo crossings with Gaza, but has not responded militarily.

Olmert said that Israel's restraint should not be misinterpreted as weakness.

"We are in favor of a genuine calm," he said while on a tour of southern Israel. "If it prevails, fine. If it doesn't, we know how to respond with full force, and in a manner that will guarantee complete security to the residents of the south."

The Israeli military said its radar detected a rocket launched from Gaza that struck near the communal farm of Mefalsim. But it said actual rocket fragments were not located.

"The closing of the border today is unjustified and another indication that the Israeli occupation is maneuvering and trying to dodge the conditions of the calm understanding," Hamas spokesman Sami Abu Zuhri said.

And Gaza's Hamas prime minister, Ismail Haniyeh, accused Israel of "backtracking on the calm."

Abu Zuhri also accused the military on Tuesday of shooting a 65-year-old Palestinian woman who lives near Israel-Gaza border in what he called "a grave violation of the calm understanding."

The military denied involvement.

Israel has taken a zero-tolerance approach to any violation of the cease-fire. Since the truce went into effect on June 19, the crossings have been closed for five days because of rocket and mortar attacks.

None of them has caused serious injuries or damage.

In its initial stage, the Egyptian-brokered truce aims to bring an end to fighting that has killed seven Israelis and more than 400 Palestinians — many of them civilians — since the Islamic militant group Hamas wrested control of Gaza a year ago. It also obligates Israel to ease its bruising blockade of the coastal strip, home to 1.4 million people.

The sanctions were designed to pressure Palestinian militants to halt their assaults on southern Israel, but they have driven ordinary Gazans even deeper into destitution and confined them to their tiny seaside territory.

In recent months, Israel's passages with Gaza have been sealed to everything but humanitarian aid and fuel supplies. Israel had committed under the truce to allow in larger shipments of some supplies, but deliveries have not increased because of the attacks.

Egypt, too, sealed its border with Gaza after the Hamas takeover, opening it only occasionally on humanitarian grounds.

On Tuesday, Egypt again opened the Rafah crossing — Gaza's main gateway to the outside world — for two days to allow hundreds of people stranded on both sides to cross.

Palestinian diplomat Nabil Amr told reporters in Cairo on Monday that Palestinians needing medical treatment and those with residency permits in Egypt or other countries abroad would be allowed into Egypt.

Fifty Gazans with medical conditions were to be the first allowed out, traveling across the border in ambulances.

Although Rafah lies on the Gaza-Egypt border, Israel has had the power to halt the crossing's operations because Europeans monitoring the passage require Israeli security clearance to operate. That clearance has not been given since the Hamas takeover, and Israel has insisted that Rafah would not "return to normal business" until Hamas releases an Israeli soldier captured two years ago.

But the emphasis on "normal business" indicated that Israel would agree to occasional openings of Rafah.

In January, Hamas blew up the border wall between Egypt and Gaza, allowing hundreds of thousands of people to move in and out of Egypt for nearly two weeks before it was resealed.