Israel bombs office of Gaza prime minister, other government buildings Updated at 8:10 a.m. ET: Israeli aircraft bombed Hamas headquarters before dawn on Saturday, including the office of Gaza Prime Minister Ismail Haniyeh, as conflict in the region entered its fourth day. Israeli planes shattered Haniyeh's office building - where he had met with Egyptian Prime Minister Hisham Qandil the day before - and struck the interior ministry. Loud explosions regularly rocked the densely populated Palestinian territory, sending plumes of smoke billowing into the sky. The occasional hiss of outgoing rocket fire showed Islamist militants were pursuing their defiance of the assault. Troops were massing on the border and witnesses said they could see Israeli ships off Gaza's coast, NBC News' Ayman Mohyeldin reported. Despite the violence, Tunisia's foreign minister arrived in the coastal enclave on Saturday in a show of solidarity, denouncing the Israeli attacks as illegitimate and unacceptable. Ammar Awad / Reuters Officials in Gaza said 41 Palestinians, among them 20 civilians including eight children and a pregnant woman, had been killed in Gaza since Israel began operations four days ago. Israel's military said its air force had hit at least 180 targets since midnight, including a police headquarters, government buildings, rocket launching squads and a Hamas training facility in the impoverished territory. Reporting from Gaza City, NBC's Richard Engel posted a message on Twitter describing the buzz of drones over the city. "It sounds like everyone is out mowing their lawns in the dark," he said. A three-story house belonging to Hamas official Abu Hassan Salah was also hit and completely destroyed early on Saturday. Rescuers said at least 30 people were pulled from the rubble. "What Israel is doing is not legitimate and is not acceptable at all," Tunisian Foreign Minister Rafik Abdesslem said as he visited Haniyeh's wrecked headquarters. It does not have total immunity and is not above international law. Hamas says it is committed to continued confrontation with Israel and is eager not to seem any less resolute than smaller, more radical groups that have emerged in Gaza in recent years. The Islamist Hamas has ruled Gaza since 2007. Israel pulled settlers out of Gaza in 2005 but has maintained a blockade of the territory. Obama calls for stability Israel launched a massive air campaign on Wednesday with the declared aim of deterring Hamas from launching cross-border rocket salvoes that have plagued southern Israel for years. The Palestinians have fired hundreds of rockets out of Gaza, including one at Jerusalem and three at Tel Aviv - Israel's commercial center. Jerusalem had not been targeted in such a way since 1970, and Tel Aviv since 1991. Although there were no reports of casualties or damage in either city, the long-range attacks came as a shock and advanced the prospect of an Israeli ground invasion into Gaza. "This will last as long as is needed; we have not limited ourselves in means or in time," Foreign Minister Avigdor Lieberman told Israel's Channel One television on Saturday. The Palestinian enclave of Gaza was attacked Friday, where the Interior Ministry took direct hits and civilians died. The two leaders also discussed options for "de-escalating" the situation, the White House said in a statement. He also called Egyptian President Mohammed Morsi on Friday, and underscored his hope of restoring stability. Israeli Prime Minister Benjamin Netanyahu held a four-hour strategy session late on Friday with a clutch of senior ministers on widening the military campaign, while other cabinet members were polled by telephone on increasing mobilization. Political sources said they decided to more than double the current reserve troop quota set for the Gaza offensive to 75,000. It did not necessarily mean all would be called up. In a further sign Netanyahu might be clearing the way for a ground operation, Israel's armed forces decreed a highway leading to the territory and two roads bordering the enclave of 1.7 million Palestinians off-limits to civilian traffic. NBC's Ayman Mohyeldin answers your questions about Israel-Gaza conflict The Israeli military said some 367 rockets fired from Gaza had hit Israel since Wednesday and at least 222 more were intercepted by its Iron Dome anti-missile system. The Israel Defense Force said Saturday that mortar fire from Gaza had damaged an electricity cable in the south of Israel. "As a result, power is out in areas of northern Gaza Strip," the IDF said in a message posted on Twitter. Four Iron Domes were deployed initially and a fifth was rushed into action on Saturday, weeks ahead of schedule. The army said it was placed in the Tel Aviv area, showing Israel's concern for the safety of its heavily populated coastline.