GAZA, June 23 (Reuters) - Israel launched air raids on Hamas security targets in Gaza on Saturday, wounding more than 20 people, medical officials in the Islamist-ruled territory said, as militants stepped up rocket fire, wounding an Israeli man.

The escalating violence threatened to unravel Wednesday's shaky Egyptian-brokered truce, which had temporarily calmed violence that erupted on Monday after a raid across Egypt's Sinai border in which an Israeli man and two gunmen were killed.

Hamas medical officials said a six-year-old Palestinian boy had been killed in an air strike but Israel's military denied any involvement.

Israel confirmed its aircraft had struck two militant targets in the Islamist-ruled territory.

Hamas medical officials said a third Israeli air raid had killed the six-year-old boy at a soccer field near the town of Khan Younis, and wounded two other people. They said a baby was wounded in a separate attack in Rafah, at the Egypt border.

Lieutenant Colonel Avital Leibovich, an Israeli military spokeswoman, denied any Israeli involvement in the boy's death, saying on Twitter that the report was the result of false rumors and that the boy had died due to an explosion of ordnance belonging to Palestinian militants.

Another Israeli military spokeswoman said she had no report of any air strikes in Rafah, where the baby was reported to have been hurt.

The Israeli strikes were reported after the worst rocket assault in six days of fighting. One projectile slammed into the Israeli town of Sderot wounding an Israeli man in the neck just as he was trying to enter a concrete shelter.

The rocket was one of more than 150 fired into Israel in the past week, the military said, after a relatively calm period. At least 15 were fired at Israel on Saturday, nearly three times as a day ago.

At least six other rockets were intercepted by an Israeli missile defense system, the military said.

Israel's military chiefs scheduled urgent consultations to weigh a course of action, a military spokeswoman said. Israeli authorities also urged many of the 1 million Israelis who live in the south to stay indoors or close to fortified shelters.

Officials in Gaza said Israel had attacked security targets in Gaza City and in northern and southern parts of the crowded coastal strip before dawn.

Nobody in Gaza claimed responsibility for the rocket fire at Israel, but a security source said the missiles had been launched by members of a fringe Salafi group sympathetic to al-Qaeda, two of whose militants were killed in Israeli raids on Friday.

Israel blamed the Salafis for Monday's cross-border raid from Egypt, after which Israel launched punitive air raids on nearby Gaza, killing 10 Palestinians, many of them militants but also including a 14-year-old boy.

Hamas militants had conditionally pledged to adhere to the truce brokered by Egypt if Israel also held fire. Israel never formally commented on the deal but its officials have pledged to respond to any rocket fire from Gaza.

Egypt has brokered such deals in the past and stepped in this time fearing the violence, which coincided with a hotly contested presidential race in Cairo, could spiral out of control. (Reporting by Saleh Salem; Writing by Allyn Fisher-Ilan; Editing by Andrew Heavens and Alessandra Rizzo)