The New York Times reported the decision by the three Middle East neighbors came as the state-run Syrian news agency said Syrian President Bashar Assad had replaced his defense minister, Lt. Gen. Ali Habib, with the army's chief of staff, Gen. Dawoud Rajh.
It was unclear why Assad made the move, the U.S. newspaper said. Habib had served in the Cabinet post since June 2009.
Withdrawing the ambassadors is the most significant action taken yet by Arab nations since protests against the Assad administration started in mid-March, the Times noted.
U.S. State Department spokesman Mark Toner told reporters Monday the United States is "very much encouraged" by the actions.
"These are further signs that the international community are repulsed -- is repulsed by the brutal actions of the Syrian government and is standing with the Syrian people. And furthermore, it's signs that, as I've said before and others have said, that President Asad and his government are further isolating themselves from the international community through their actions," Toner said.
Saudi Arabia demanded an end to the bloodshed the Syrian military has inflicted on opposition activists, including nearly 70 people killed in a single day of military assaults.
"What is happening in Syria is not acceptable for Saudi Arabia," King Abdullah said in a statement read over the Saudi-owned al-Arabiya news channel.
"Syria should think wisely before it's too late and issue and enact reforms that are not merely promises but actual reforms. Either it chooses wisdom on its own or it will be pulled down into the depths of turmoil and loss," said Abdullah, whose own monarchy bans political opposition and supplied troops to neighboring Bahrain to repress anti-government protests.
"The kingdom does not accept the situation in Syria because the developments cannot be justified," Abdullah said, insisting Damascus introduce "comprehensive and quick reforms."
"The future of Syria lies between two options -- either Syria chooses willingly to resort to reason, or [it will] face being swept into deep chaos, God forbid," he said.
Abdullah's rebuke and warning followed an increasingly widespread surge of condemnation of Assad Sunday, from the Arab League to the pope.
The 22-member league, which had been silent in the five months since the uprising began, said Sunday it was "alarmed" by the bloodshed. It called on Syrian authorities to stop attacking protesters and demanded an immediate halt to the violence.
A league appeal in March for international intervention in Libya laid the groundwork for NATO's bombing campaign against leader Moammar Gadhafi. But with Syria, the league has so far specifically asked the West to stay out.
Pope Benedict XVI called on Assad to respond properly and adequately to the Syrian people's "legitimate aspirations," adding he had "deep concern" about "Syria's dramatic and increasing episodes of violence," which he said had led to "numerous victims and grave suffering."
CNN reported Monday hackers accessed the Web site of Syria's Defense Ministry replacing its content with anti-government messages and support for the protests.
More than 250 Syrian tanks and armored vehicles Sunday laid waste to the country's biggest northeastern city and oil capital, Deir el-Zour, which has been under siege for days, in a predawn offensive that also included snipers positioned on rooftops picking off "anything that moves," said the activist Local Coordination Committees, which tracks the uprising and organizes some protests. Nearly 50 people were killed there, activists said.
Thousands fled the city of about 511,000, the activists said. A family of six trying to escape -- a couple with four children -- were among the dead, they said.
Syrian tanks also shelled Houleh, a town in central Syria's Homs province that had also seen large protests, killing about 20, the activists said.
More than 300 people have died in the past week, the bloodiest in the five-month uprising, the activists said.