U.S. President Barack Obama on Monday talked with Israeli Prime Minister Benjamin Netanyahu over phone, expressing "deep regret" over casualties in Israeli raid on Gaza aid.

"He said he understood the Prime Minister's decision to return immediately to Israel to deal with today's events. They agreed to reschedule their meeting at the first opportunity," a White House statement said.

"The President expressed deep regret at the loss of life in today's incident, and concern for the wounded, many of whom are being treated in Israeli hospitals," the statement said.

"The President also expressed the importance of learning all the facts and circumstances around this morning's tragic events as soon as possible," the statement added.

Israeli forces on Monday attacked the aid ships when trying to stop the flotilla from reaching Gaza, killing up to 20 and wounding more than 30 others.

Netanyahu has decided to cut short his trip to Canada and the United States and fly back to Israel to deal with the aftermath of the incident.