The military wing of Gaza's rulers Hamas on Thursday vowed revenge after Israeli strikes killed three militants in the latest flare-up of violence.
Israel said the artillery fire late on Wednesday was in retaliation for shots fired at troops along the border which injured one soldier.
"We announce the raising of the preparedness level to the highest and alert all of our soldiers and forces in every location," said a statement from the al-Qassam Brigades, the military wing of Hamas.
"The enemy shall pay a high price in blood for the crime which it commits daily against the rights of our people and our fighters."
The Gaza border was quiet on Thursday morning, however.
The three men were killed on Wednesday by Israeli artillery fire targeting a Hamas military base near the border east of Gaza City.
In total the Israeli army said it hit seven Hamas sites throughout Gaza in response to the border shooting.
The military said nine projectiles -- likely mortars or rockets -- were fired at Israel from Gaza overnight, including one that was intercepted by air defence systems.
It said most were fired at "open territory adjacent to the security fence". No injuries or damage were reported.
The latest bloodshed came five days after the UN and Egypt brokered a deal to halt a July 20 surge in violence.
Tensions have spiked since Palestinians began protesting along the Gaza border in late March, with Israeli fire killing at least 153 Palestinians since then.
Most were shot dead during protests and clashes but others were killed in Israeli airstrikes or artillery fire.
One Israeli soldier was killed by sniper fire from Gaza, setting off a wave of Israeli strikes on July 20.