Israel said the decision was made after a mortar landed near the town of Sderot in the western Negev region late Monday. All but one of the Gaza border crossings are currently shut, once again halting supplies of food and other essential items to the Palestinian enclave.
"By the Defense Minister decision, today all crossings except for the Erez checkpoint, which can be used by Palestinians in case of acute humanitarian necessity, are closed," the spokesman said.
Hamas has denied involvement in the mortar attack.
The six-month truce between Israel and Hamas-led militants came into force on June 19 after Egypt was involved in the ceasefire mediation between the two sides. The deal is limited to the Gaza Strip.
Under the agreement Israel was supposed to gradually abolish economic sanctions imposed on Gaza, and the sides should have continued to negotiate the release of the kidnapped Israeli soldier Gilad Shalit, who has been held captive in Gaza for two years.
Israeli Prime Minister Ehud Olmert earlier said the ceasefire would be "very short."
