GAZA CITY, Palestinian Territories - Israeli jets launched fresh airstrikes in Gaza on Thursday night and Friday morning after militants in the Palestinian territory again set off incendiary balloons into southern Israel, the army and media reports said.

The fire balloons and airstrikes are the latest outbreak of violence which is putting pressure on a fragile cease-fire between Israel and Gaza's Hamas rulers that came into place on May 21, ending 11 days of heavy fighting.

"Over the past day, arson balloons were launched from the Gaza Strip into Israeli territory," Israel's military said in a statement. "In response, fighter jets struck military compounds and a rocket launch site belonging to the Hamas terror organization."

Some journalists in the Palestinian enclave also reported hearing explosions, which the army said hit sites in Gaza City and in Khan Yunis, in the south of Gaza, and home to around 2 million people.

Soon after the strikes, Hamas militants opened fire with heavy machines guns toward the Jewish state, as Israeli warning air raid sirens rang out.

Palestinian militants in Gaza launched balloons for a third day running on Thursday, according to Israeli firefighters battling the blazes sparked by the devices.

The balloons are intended to set fire to farmland and bush surrounding Gaza.

After the first wave of fire balloons sparked blazes on Tuesday, Israel's military launched a retaliatory wave of strikes early Wednesday.

Then, as the balloons continued coming in, the air force launched a second round of strikes overnight Thursday to Friday.

Under new government

This week's airstrikes on Gaza were the first under Israel's new government headed by Naftali Bennett, whose ideologically disparate coalition on Sunday ousted long-serving leader Benjamin Netanyahu.

Israel army chief Aviv Kohavi late on Thursday issued orders to "increase the IDF's (Israeli Defense Forces') readiness and preparedness for a variety of scenarios including a resumption of hostilities", the army statement read.

US Secretary of State Antony Blinken said he had spoken with Israel's new Foreign Minister Yair Lapid to offer support. "We discussed America's unwavering commitment to Israel's security, our bilateral relationship, and the challenges ahead," Blinken tweeted after the Israeli strikes.

They also discussed "the need to improve Israeli-Palestinian relations in practical ways," State Department spokesman Ned Price said in a separate statement that did not mention the airstrikes.

The new exchanges come as Egypt tries to consolidate the ceasefire it helped broker.

The conflict killed 260 Palestinians including some fighters, the Gaza authorities said. In Israel, 13 people were killed, including a soldier, by rockets fired from Gaza, the police and army said.

Cairo, along with the United Nations, is hoping to support the reconstruction of Gaza following a war that saw whole tower blocks reduced to rubble and key infrastructure smashed.

The densely crowded Mediterranean enclave has been under an Israeli blockade since 2007.