Israeli airstrikes in the Gaza Strip have hit a Hamas government building.

The Cabinet headquarters was among targets hit early Saturday in Gaza. There are no initial reports of injuries.

The strikes followed an exchange of rocket fire Friday between Israel and Palestinian Hamas militants in Gaza with one Palestinian rocket landing in a field outside Jerusalem.

Air raid sirens wailed through Jerusalem Friday after the unprecedented attack on Jerusalem, but Israeli officials said there were no injuries or damage. Hamas claimed responsibility.

Israel responded by calling up thousands more military reserves for a potential force of 75,000 reservists in preparation for a possible ground incursion into Gaza. It also shut down the main roads around the Gaza Strip.

The situation between Israel and Hamas threatens to explode into all-out war.

Egyptian Prime Minister Hesham Kandil visited Gaza, promising Egypt “will make sacrifices and every effort to achieve a truce.” But in Cairo, Egyptian President Mohamed Morsi lashed out at what he called unacceptable Israeli aggression.

U.S. President Barack Obama telephoned the Egyptian president Friday to discuss the situation and commend Egypt's efforts to defuse the tensions.

Mr. Obama also spoke with Israeli Prime Minister Benjamin Netanyahu. The White House says Mr. Netanyahu thanked the United States for helping Israel develop a missile defense system called the Iron Dome. The prime minister says the system has knocked out hundreds of rockets, saving countless Israeli lives.

An Israeli missile Wednesday killed the top military commander of Hamas in Gaza. Israel says the killing was in direct response to the almost daily rocket attacks on Israel from Gaza.

Hamas vowed revenge and warned that Israel had opened up what it calls “the gates of hell.”