The State Department warned U.S. citizens on Monday against traveling to Israel, the West Bank and Gaza, citing the fighting between Israeli forces and Hamas.

"The Department of State recommends that U.S. citizens consider the deferral of non-essential travel to Israel and the West Bank and reaffirms the longstanding strong warning to U.S. citizens against any travel to the Gaza Strip," the State Department said, adding the warning replaced a previous one issued on Feb. 3.

"The security environment remains complex in Israel, the West Bank, and Gaza, and U.S. citizens need to be aware of the risks of travel to these areas because of the current conflict between Hamas and Israel," the statement added.

The White House put pressure on Israel on Monday to take greater steps to protect Palestinian civilians from being killed in an Israeli offensive in the Gaza Strip.

President Barack Obama, who repeated his position that Israel had the right to defend itself against rocket and tunnel attacks from Hamas militants, expressed serious concern over the number of casualties on both sides.

The Palestinian death toll rose above 500 on Monday while Hamas, which killed 13 Israeli soldiers in Gaza on Sunday in the biggest one-day toll in eight years, continued to fire rockets deep into Israel.

White House spokesman Josh Earnest said Israel could do more to follow its own criteria for protecting innocent people.

"Israel must take greater steps to meet its own standards for protecting civilians from being killed, and we'll continue to ... send that message directly to the Israelis," Earnest told reporters.

Earnest made clear that the United States believed Israel faced a significant threat from Hamas. It was "unacceptable" for Hamas to continue firing rockets at Israeli civilians, he said.

Obama, in separate comments to the media, said Israel had made significant dents in the militant group's tunnels.

"As I've said many times, Israel has a right to defend itself against rocket and tunnel attacks from Hamas. And as a result of its operations, Israel has already done significant damage to Hamas's terrorist infrastructure in Gaza," Obama told reporters at the White House.

"I've also said, however, that we have serious concerns about the rising number of Palestinian civilian deaths and the loss of Israeli lives, and that is why it now has to be our focus and the focus of the international community to bring about a ceasefire that ends the fighting and that can stop the deaths of innocent civilians," he said.

Secretary of State John Kerry, who arrived in Egypt in an effort to help broker a ceasefire, appeared to show his own frustration with Israel's operation in Gaza during a conversation caught on an open microphone before an interview on "Fox News Sunday."

"It's a hell of a pinpoint operation," Kerry was overheard saying sarcastically, referring to the Israeli offensive.