Israel launched air strikes in the Gaza Strip on Monday, wounding at least 19 people, after militants fired mortar shells and rockets into the Jewish state, witnesses and militant groups said.

The number of raids and casualties in one evening showed the rising tension between Israel and Hamas, the Islamist group that controls Gaza.

Hamas has stepped up rocket salvoes into Israel after a hiatus since the two sides fought a war two years ago, claiming responsibility for firing more than two dozen mortar shells and rockets at the weekend.

The Israeli military said its warplanes fired at six targets on Monday in response to a rise in rocket and mortar attacks on Israel. About 130 such attacks had been made on Israel this year, 56 of them since Saturday, a military spokesman said.

He said the military held Hamas "solely responsible for terrorist activity in the Gaza Strip and warns Hamas not to continue its aggression".

Hamas medical officials said 19 people were wounded in the Israeli air strikes, including four militants, seven children and two women.

Palestinian analysts linked the growing violence to calls for President Mahmoud Abbas to heal a four-year rift with Hamas, which seized control of Gaza in a bloody 2007 struggle with Abbas's Western-backed Fatah movement.

Militants in Gaza often fire rockets at Israel but Hamas itself had avoided doing so or claiming responsibility for such attacks in recent months.

Abbas said last week he was willing to visit Gaza for talks designed to reunify Palestinian ranks.

Some Hamas officials fear a reconciliation with Fatah could threaten the Islamists' hegemony in Gaza. Israel has signalled it would see such reconciliation as a threat, given Hamas's refusal to recognise its existence and join peace talks.

"I think the escalation from the Hamas side is calculated," Talal Okal, a Palestinian expert, said in an interview, adding that he believed Israel had similar motives, although both sides may try to avoid a wider conflagration.