US President Barack Obama Sunday called Israeli Prime Minister Benjamin Netanyahu, expressing his "serious concern" about growing number of casualties in the Gaza crisis, a White House statement said.Obama and Netanyahu spoke Sunday morning over phone, their second call in three days to discuss the situation in Gaza, Xinhua reported. Obama reiterated the US' condemnation of attacks by Hamas against Israel, and reaffirmed Israel's right to defend itself, the statement said."The president also raised serious concern about the growing number of casualties, including increasing Palestinian civilian deaths in Gaza and the loss of Israeli soldiers," it said.Obama told Netanyahu that US Secretary of State John Kerry would soon travel to Cairo to seek an immediate cessation of hostilities based on a return to the November 2012 ceasefire agreement, the statement said. The US president underscored that the US will work closely with Israel and regional partners on implementing an immediate ceasefire.