In a Sunday phone call with Israeli Prime Minister Benjamin Netanyahu, President Barack Obama reaffirmed Israel’s right to defend itself, but the President expressed “serious concern” for the rising casualty count in the Gaza Strip. Obama’s concern applies to Palestinian civilians and Israeli soldiers alike, according to a White House readout of Sunday’s call.

Obama, who has previously condemned attacks on Israel by Hamas, also said that U.S. Secretary of State John Kerry will travel to Cairo to “seek an immediate cessation of hostilities” based on a 2012 ceasefire deal.

The call, the two leaders’ second in three days about Israel’s latest offensive in Gaza, occurred not long after Israeli military operations on the ground escalated in Gaza. At least 87 Palestinians and 13 Israeli soldiers were killed Sunday, according to Palestinian health officials and the Israeli military.

Write to Nolan Feeney at nolan.feeney@time.com.