Egypt and Hamas said earlier the ceasefire would come into effect on Thursday.
The truce aims to end rocket and mortar bomb attacks on Israel from Gaza, as well as halt Israeli raids and air strikes in the Palestinian territory.
However Israel itself has yet to confirm its agreement to the deal.
Speaking to reporters, Casey denied suggestions that the deal showed the US was losing its importance as a player in Middle East peace efforts.
"I think the US role in this has been important, I think it is a continuing one," he said.
He said Condoleezza Rice, the US secretary of state, had spoken previously "about the importance of establishing calm and the appreciation we have for Egypt's efforts to facilitate peace between the Israelis and the Palestinians in a general way".
News of the truce came shortly after Rice returned to Washington following her sixth visit to the region this year to try to push Israel and Mahmoud Abbas, the Palestinian president, closer to a peace deal.
At the White House, spokesman Tony Fratto said he had no comment on the reported Israel-Hamas truce, saying there had not been anything official.
"We really want to see details of any kind of agreement and what the views are of people in the region before we comment on it," he said.