President Bush says he is optimistic about the future of Iraq, but warns of "difficult and exhausting" days ahead.
Mr. Bush said in a speech Friday that it was a "moment of choosing" for the Iraqi people, and urged restraint amid sectarian violence that has claimed scores of lives since Wednesday.
He said the situation in Iraq remains serious, and the next days are expected to be "intense."
Mr. Bush said the United States will do everything possible to help the Iraqi government bring to justice the perpetrators of Wednesday's bombing of the Shi'ite Askariya shrine in Samarra and the subsequent attacks.
Secretary of State Condoleezza Rice, en route to Shannon, Ireland, said there is concern that foreign forces are stoking sectarian tensions in Iraq -- and may try to do so in other parts of the region.
She said it is not known who carried out the Samarra bombing, but said it is some force that wants to destroy the basis for a national unity government.
In the wide-ranging speech to U.S. military veterans, Mr. Bush also called on the Palestinian militant group Hamas, which won parliamentary elections in January, to embrace democracy and recognize Israel.
He also said the international community will not allow Iran to develop nuclear weapons.