Although the statement gave no further details, Israeli military affairs correspondents who are briefed regularly by the armed forces said it was not a ground invasion, and that troops were firing artillery from Israel's side of the border.
Trouble loomed on another front after three rockets launched from Lebanon crashed into the Mediterranean off Israel's northern coast late Wednesday, raising the specter of a second battleground with Iran-backed Lebanese Hezbollah militants.
Prime Minister Benjamin Netanyahu vowed to continue the operation, saying in a video statement that Israel would "extract a very heavy price from Hamas". "The last word has not yet been uttered, and this operation will continue as long as necessary".
The deaths and devastation from the fighting mounted on Thursday.
Since Monday, militants have fired more than 1,050 rockets from Gaza, according to the Israeli military, and Israel has conducted hundreds of strikes in the tiny territory where 2 million Palestinians have lived under a crippling Israeli-Egyptian blockade since Hamas took power in 2007.
Labour's Imran Hussein said: "The silence of the worldwide community is deafening even as the Palestinians scream out for help".
The UN Security Council said it will publicly discuss the escalating situation on Sunday.
The conflict threatens to deteriorate into an all-out war that could draw in other regional players.
Zilberman said Israel was "building up forces on the Gaza border", a deployment that raised speculation about a possible ground invasion, a move that would recall similar incursions during Israel-Gaza wars in 2014 and in 2009.
Earlier Thursday, chief Israeli military spokesman, Brig. "We'll activate them when we deem it appropriate".
The fighting broke out late Monday when Hamas fired a long-range rocket at Jerusalem in support of Palestinian protests there against the policing of a flashpoint holy site and efforts by Jewish settlers to evict dozens of Palestinian families from their homes.
Johnson apologises ‘unreservedly’ for ’71 attack in N. Ireland
John Teggart, whose father was killed at Ballymurphy, said the government's public apology should have come from Johnson.
"Whether they are rocket attacks or indiscriminate acts that fuel the cycle of violence and bloodshed - they are also never justified".
The current round has roots in tensions that have been festering since the beginning of the Muslim fasting month of Ramadan in April.
The stepped-up fighting came as communal violence in Israel erupted for a fourth night, with Jewish and Arab mobs clashing in the flashpoint town of Lod.
The violent clashes between Arabs and Jews in Jerusalem and other mixed cities across Israel has meanwhile added a new layer of volatility to the conflict not seen in more than two decades.
Street battles have spread across the country, with marauders attacking people, synagogues, businesses and vehicles. An Arab man was fatally shot, and there have been attacks on Arab-owned cars. However, multiple outlets are reporting no ground troops went inside Gaza.
The communal strife has torn at the uneasy coexistence between Israeli Arabs and Jews.
These include Washington's recognition of disputed Jerusalem as Israel's capital, a US plan to end the conflict that they saw as favourable to Israel, and continued settlement building. The advisory also warned against travel to Gaza.
"This is the result of long-term simmering of discrimination and all kinds of grievances", said Avraham Sela, a Hebrew University political scientist.
"It will take more time, but. we will achieve our goal - to restore peace to the state of Israel", he said, according to the Associated Press.