Hamas militants fired dozens of mortar shells from Gaza into southern Israel during a 15-minute period on Saturday morning, slightly injuring two Israeli civilians and sharply escalating tensions along the Israel-Gaza border.

The Israeli military responded to the unusually intense barrage with tank and helicopter fire. Gaza officials reported that five Palestinians were wounded by Israeli fire directed at a Hamas security facility east of Gaza City, including three security officers and two civilians, one of them a child.

In an uncommon step, the military wing of Hamas, the Islamic group that controls Gaza, took responsibility for the mortar fire. Although it has allowed smaller groups to carry out sporadic attacks against Israel, Hamas has largely maintained a cease-fire since Israel’s devastating three-week military offensive in Gaza that ended in January 2009 and that came after years of persistent rocket and mortar fire against southern Israel.

The Hamas military wing said in a statement that it had fired 33 mortar shells, though the Israeli police put the number at closer to 50. And while Hamas said that it was firing at Israeli military bases along the border, mortar fire is notoriously inaccurate, and the Israeli side of the border is dotted with villages and communal farms.

An Israeli military spokeswoman, speaking on condition of anonymity under army rules, said the mortar shells fell all along the border, but could not specify where for security reasons. The Israeli news Web site Ynet showed pictures of a damaged house in one of the communities and of another mortar shell that fell near a playground.

Hamas said that it was retaliating for Israeli attacks on Gaza. On Thursday, two Hamas militants were killed in an Israeli airstrike on a training camp. The Israeli military said the strike was in response to the firing of a military-use projectile from Gaza into southern Israel earlier that morning.

The show of force by Hamas militants came amid talk by the rival Palestinian leaderships of the West Bank and Gaza about reviving efforts toward internal unity. There has been increasing public pressure to end the political division between Hamas-run Gaza and the Fatah-dominated Palestinian Authority in the West Bank. Hamas won parliamentary elections in 2006 and then, after a period of factional fighting, routed Fatah forces in Gaza and took full control in June 2007.

Saturday’s escalation raised the possibility that the Hamas military wing opposes the tentative moves toward unity that were welcomed by the local political leadership in Gaza. It also came against the background of Israel’s capture on Tuesday of a shipment of weapons that Israel says were bound for Gaza. Israel found the weapons on board a merchant ship intercepted by its naval forces en route from Turkey to Egypt. The weapons included more than 2,000 mortar shells and six advanced anti-ship missiles.

Also Saturday, a group of about 10 men suspected of being Hamas security officers broke into several media offices in Gaza City, chasing down journalists who had filmed Hamas police officers forcefully dispersing a peaceful demonstration for Palestinian unity.

The intruders, armed with handguns and batons, ransacked the Reuters bureau, smashing a television and camera tripods. They struck one Palestinian Reuters journalist on the arm with a metal bar and threatened to throw another out of the window of the high-rise building, Reuters reported.

They also broke into the offices of CNN, Japan’s NHK station and Mayadeen Media Group, a local production company, where they beat a cameraman with a club on his head. They confiscated videotapes and cameras from some of the media companies.

The offices that were broken into overlook the Square of the Unknown Soldier, where the demonstration took place.

The men told Reuters journalists that they were from the internal security services of Hamas, but they showed no documents, Reuters reported.

Fathi Hammad, Hamas’s interior minister, met a delegation of journalists and promised to investigate the attacks. He denied that the assailants were from Hamas.