As the southern border continued to heat up Thursday, with intermittent rocket fire striking southern Israel, residents were advised to stay within 15 seconds of bomb shelters.

Over 15 rockets were fired from the Gaza Strip Thursday evening, leaving one soldier lightly injured.

Palestinian sources said Israel had issued a 48-hour ultimatum to Hamas to halt the fire or face a massive Israeli strike. The ultimatum was conveyed to Hamas leaders via Egyptian intelligence, they said.

An hour before the evening rocket barrage, Hamas said that in the event of an escalation, Israel would “be surprised” by its rocket arsenal and range.

“We promise that one stupid move your leaders make will constitute sufficient ground to turn all of your towns, even those you wouldn’t expect, into targets and burning cinders,” said Abu Ubaida, a spokesman for the Izz ad-Din al-Qassam Brigades, Hamas’s armed wing. Israel may initiate the escalation, “but it doesn’t know how it will continue and how it will end,” he said.

“The threats the occupiers issue, and the allusions to war against Gaza, are threats that have no meaning in our dictionaries, other than drawing the hour of vengeance and difficult lesson-learning closer,” Ubaida added.

He said that Israel’s move to rearrest — during an 18-day operation to find three kidnapped Israeli teens (their bodies were found in the West Bank earlier this week) — prisoners released during the 2011 swap for IDF soldier Gilad Shalit “crossed a line and we won’t be silent about it.”

The IDF beefed up its ground forces around the Gaza Strip on Thursday, as tensions continued to rise along the southern border region; and in East Jerusalem, where the recent killing of a Muslim teenager, in an alleged revenge attack over the killings of the Israeli teens, triggered widespread riots on Wednesday.

But the move came in conjunction with unusually soothing messages from the army. “We want to deescalate the situation and restore calm,” said Lt. Col. Peter Lerner, who described the deployment as defensive in nature.

The region has been increasingly tense since the June 12 kidnapping of the three Israeli teens and the onset of the holy month of Ramadan.

Hamas, which has apparently taken part in the rocket fire recently for the first time since 2012, failed in its attempt to kidnap and trade the Israeli teens for Palestinian prisoners, Lerner said, and therefore has been “pushed into a corner.”

In the West Bank, he added, the army’s current strategy comprises three main components: finding those responsible for the killing of Eyal Yifrach, Gil-ad Shaar, and Naftali Fraenkel; finding those who killed Muhammed Abu Khdeir, the 16-year-old boy who was abducted from his hometown of Beit Hanina on Wednesday; and avoiding violence on the first Friday of Ramadan.

IDF Chief of Staff Lt. Gen. Benny Gantz, in advance of Friday’s mass prayer gatherings, has instructed all Central Command troops to “limit points of friction,” Lerner said.

Israel’s cabinet, meanwhile, has remained mum on possible anti-Hamas operations in the wake of the kidnapping and murder of the three Israeli teens by Hebron-based Hamas members.

Whether or not a larger IDF operation is imminent, the build-up is a message to Hamas — under pressure from the shuttering of its border with Egypt, a multi-year siege on its Israeli border and a collapsing economy in the Strip — that escalation could spell significant damage for Gaza and its rulers.

Rocket fire from Gaza damaged two buildings in Sderot on Thursday morning. No injuries were reported. One of the rockets hit the side of a building that contains a preschool, but did not explode. The area was closed off to passersby, police spokesman Micky Rosenfeld tweeted on Thursday morning, as police sappers removed the unexploded warhead.

Israel’s Iron Dome system shot down two rockets fired from Gaza fired in the direction of the southern town of Netivot early Thursday morning.

Times of Israel staff contributed to this report.