Four Israeli soldiers have been hurt, two of them seriously, in an explosion near the Israeli-Gaza border.
The army said a Palestinian flag was flying in the area, and when the troops approached they were hit by the blast.
In response, Israel conducted air strikes against Hamas targets in Gaza. There were no reports of casualties.
Israeli media are describing the blast as the worst incident on the border since Israel and Hamas militants, who dominate Gaza, fought a war in 2014.
No group has so far said it was behind the explosion.
The incident happened at 16:00 local time (14:00 GMT) east of the town of Khan Younis.
The army said the explosive device had been planted during a demonstration there on Friday and was attached to a flag.
The troops were approaching from the Israeli side when the device detonated.
Israeli Prime Minister Benjamin Netanyahu, who is attending a security conference in Munich, Germany, said: "The incident on the Gaza border is very serious. We will respond appropriately."
The Israeli army said fighter jets responded with strikes on six Hamas military targets, including a tunnel near Zeitun being dug by militants towards Israel and compounds near Deir al-Balah and Khan Younis.
Palestinian officials said three Hamas training camps and one belonging to a smaller group had been struck but no-one was injured.
Israeli media also said a rocket from Gaza fell near a house in the south of the country on Saturday evening. There were no casualties.
Israel holds Hamas responsible for all rocket and mortar fire from the territory. Hamas has fought three wars with Israel since 2008.
Correspondents say the border area has been generally quiet in the last few years but there has been an increase in violence since US President Donald Trump's announcement in December recognising Jerusalem as Israel's capital.
Israel regards Jerusalem as its indivisible capital. Palestinians want the east of the city, which Israel occupied in the 1967 Middle East war, to be a capital of a future state.