Gaza's borders have been mostly quiet for the first time in months
The Israeli army says Palestinians have fired a rocket over the border from the Gaza Strip in violation of a week-old ceasefire agreed between the two sides.
The militant al-Aqsa Martyrs Brigades said they fired the rocket, which fell harmlessly, to retaliate for alleged Israeli violations of the truce.
Gaza's Hamas rulers have called on all groups to respect the ceasefire but say they will not be Israel's "police".
Palestinian militants accused Israel of shooting at farmers and fishermen.
The militants also say Israel has been closing the cargo crossings into Gaza in violation of the ceasefire terms.
A rocket attack on Wednesday, which hit the Israeli town of Sderot, was claimed by Islamic Jihad militants as retaliation for an Israeli raid in the West Bank, although the truce officially applies only to Gaza.
Israel says it will keep its military crossings into Gaza closed while rocket attacks continued.
Meanwhile, Israeli and Egyptian officials were holding talks about Gaza in Cairo.
Israel, represented by envoy Ofer Dekel, wants to renew indirect talks on the release of Israeli soldier Gilad Shalit held in Gaza since the summer of 2006.
Hamas wants Israel to release hundreds of Palestinian prisoners in exchange for freeing Cpl Shalit.
However, Israel has previously rejected many of the names Hamas put forward.
The Israeli military had been allowing more imports into Gaza since the truce was agreed, but the border was closed again after Tuesday's rocket fire which officials called a "grave violation" of the truce agreement.
Israeli military officials had been quoted as saying goods crossings would open on Friday, but that was before the latest attack.
The Erez pedestrian crossing remained open to diplomats and journalists.
A Hamas spokesman said continuing the closure would render the agreement for calming the situation in Gaza "meaningless".
"Securing the continuation of the Palestinian factions' commitment to the deal hinges on the occupation's lifting of the siege and the opening of all the crossings in the first 10 days," he said.
No injuries were reported in either attack.
The Israeli government had previously warned it would respond with considerable force if any of the Palestinian groups resumed violence.
The Egyptian-brokered truce between Israel and Hamas in Gaza started on 19 June and is supposed to last six months.
It is designed to halt Israeli incursions into the Gaza Strip, and to stop rockets and mortars being fired from Gaza into southern Israel.
Israel tightened its restrictions on travel in and out of Gaza last year, sharply cutting back on supplies into the impoverished and crowded territory after Hamas group took control from the Fatah-controlled Palestinian Authority.