The White House called on Israel on Wednesday to exercise "maximum restraint" in Gaza and said it was very concerned about the number of Palestinian deaths after Israeli forces opened fire on a protest march.
The White House said it has asked the Israeli government to explain its actions, which resulted in the deaths of at least 10 Palestinians and raised the two-day death toll to 33 in Israel's bloodiest raid in Gaza in years.
The criticism underscored growing US concerns about Israeli actions one day after President George W. Bush called the violence "troubling" and the White House said it received assurances from the Israeli government about the military operation.
"We are very concerned about reports from Gaza and the number of Palestinians who are said to have been injured and killed," White House spokesman Scott McClellan said.
"We have asked the government of Israel for the facts about what happened today. We will continue to follow this closely and we urge all parties to exercise maximum restraint," McClellan said.
He said the administration's response reflected the latest developments in Gaza, where 1.3 million Palestinians live in poor conditions and with little freedom of movement.
The Palestinian Authority has branded the Israeli attack a "war crime" and demanded international protection for the Palestinian people.
The raid has raised an international outcry because of Israeli threats to flatten hundreds of homes to widen an army-controlled security corridor along the border with Egypt.