TEL AVIV — Hamas has launched an effort to accelerate missile production in the Gaza Strip.

Israeli military sources said Hamas has used Israel's ceasefire pledge to rebuild its military capabilities. The sources said this includes production of the Kassam-class short-range missiles, anti-tank rockets and mines.

At the same time, Hamas was said to have restored strongholds destroyed by Israel's military. The sources said the strongholds have included weapons factories and warehouses.

"Hamas and other groups are preparing for another offensive," an Israeli military source said. "They are being encouraged and paid by Iran to stage provocations that would end the ceasefire."

On Thursday, Palestinian gunners fired at least 33 mortars, anti-tank rockets and Kassam-class short-range missiles toward Israeli communities and military outposts in the central Gaza Strip, the sources said. The mortars struck homes and greenhouses, but nobody was injured.

Hamas claimed responsibility for the mortar attacks, saying they were launched to avenge the killing of a Palestinian by Israeli soldiers. In a statement, Hamas, disavowing a PA ceasefire declaration, said it fired 46 mortars and rockets on early Thursday.

"We have never promised a ceasefire," Hamas spokesman Mushir Masri said.

"Whoever declared such a ceasefire is committed to this ceasefire." As Hamas launched mortar strikes against Israeli communities, about 30 Hamas insurgents stormed a PA prison in Gaza City and released several inmates in a battle in which a PA officer was killed. Later, PA forces raided a Hamas stronghold, an operation that sparked another armed clash.

Israel's military did not respond to the mortar attacks, which continued throughout Thursday. The military maintained border terminals for the passage of Palestinians and cargo while Israeli officials complained of Hamas attacks to Egypt and the United States.

Two days earlier, Israel and the Palestinian Authority announced a mutual ceasefire in the West Bank and Gaza Strip. Over the past day, several Palestinian attacks and attempted strikes were reported against Israeli troops.

But Israeli sources said PA Chairman Mahmoud Abbas has not moved against Hamas's buildup. They said Abbas does not consider Hamas military preparations or weapons smuggling as violations of his ceasefire pledge. Abbas has sent an envoy to Beirut to meet Hizbullah in an effort to win its cooperation for a ceasefire.

Israel's military and police have been placed on high alert for a suicide attack by Palestinian insurgency groups. The sources said one alert stemmed from a suspected Palestinian plot to blow up the Karni terminal in the eastern Gaza Strip.