JERUSALEM — Israel and the militant group Hamas seemed set on a collision course on Tuesday, with an escalation of cross-border clashes around the Gaza Strip, Hamas vowing to avenge the deaths of six of its fighters, and Israeli warplanes attacking dozens of targets in the Palestinian coastal territory.

Lt. Col. Peter Lerner, a spokesman for the Israeli military, said that the army was completing the deployment of two infantry brigades along the border with Gaza and that the government had approved the call-up of 1,500 reservists, mainly Home Front Command and aerial defense units. “If last week we were talking about calm being answered by calm,” Colonel Lerner said, “we are now talking about preparing for an escalation.”

Early Tuesday, the Israeli Defense Forces announced on Twitter that they had “commenced Operation Protective Edge in Gaza against Hamas in order to stop the terror Israel’s citizens face on a daily basis.”

According to a Reuters report, predawn airstrikes had hit at least 30 targets in Gaza, including homes of suspected militants.