Obama also dismissed Syrian President Bashar Assad's move to scrap the emergency rule imposed by the ruling Baath Party when it seized power in 1963 and allow for peaceful demonstrations as "not serious" in light of the violence against protesters.
"The United States condemns in the strongest possible terms the use of force by the Syrian government against demonstrators. This outrageous use of violence to quell protests must come to an end now," Obama said in a statement.
"We call on President Assad to change course now, and heed the calls of his own people."
He denounced the Assad regime's use of force and "outrageous human rights abuses," saying it had chosen to reject the rights and aspirations of the Syrian people.
"Instead of listening to their own people, President Assad is blaming outsiders while seeking Iranian assistance in repressing Syria's citizens through the same brutal tactics that have been used by his Iranian allies," Obama added.
"We strongly oppose the Syrian government's treatment of its citizens and we continue to oppose its continued destabilizing behavior more generally, including support for terrorism and terrorist groups."
Syrian protesters (Photo: AP)
US diplomats leveled similar accusations last week amid reports that Tehran was providing Syria with equipment to put down anti-government protests and monitor opposition groups, in addition to technical assistance to monitor online communication from opposition groups to organize protests.
Obama's comments came after activists and rights groups said the bloodiest day in over a month of protests saw Syrian government forces kill at least 72 people when they opened fire on demonstrators, seeking to disperse thousands who took to the streets for "Good Friday" protests.
The latest deaths brought to nearly 300 the number of people killed in pro-democracy protests since mid-March.
Tens of thousands of people took to the streets across the country a day after Assad scrapped the decades-old emergency rule.
But his forces fired live rounds at demonstrators in several towns and cities nationwide, witnesses and activists said.
White House spokesman Jay Carney had earlier called on Bashar's regime to "cease and desist from the use of violence."
"We call on the Syrian government to follow through on its promises and take action toward the kind of concrete reform that they promised," he added.
The official SANA news agency said security forces intervened using only tear gas and water cannon to "prevent clashes" between protesters and passers-by.
Assad, in power since replacing his father Hafez as president in 2000, issued decrees Thursday scrapping emergency rule, abolishing the state security court and allowing citizens to hold peaceful demonstrations.
But his detractors said the moves were not enough, and the so-called Syrian Local Coordinating Committees of protesters made a raft of demands, urging a halt to the torture, killings and arrests of protesters.
Also Saturday, UN Secretary General Ban Ki-moon denounced Syria's actions and called for a "transparent and independent" inquiry of the killing of protesters.
According to spokesman Farhan Haq, Ban called for attacks against "peaceful protesters" to cease "immediately", after Holy Friday's outbursts of violence caused over 80 victims.
Ban's call followed that of six Syrian human rights, including the National Organization for Human Rights in Syria, which demanded that the government to set up a judicial investigation committee to try those who shot at protesters and to release all political detainees.
Friday's protests came after a call by Facebook group The Syrian Revolution 2011 for rallies spanning the Christian and Muslim faiths on "Good Friday," which commemorates Jesus Christ's crucifixion.
Assad's scrapping of emergency rule and abolition of the state security court was the latest in a series of concessions over the past month to placate protesters.
Before Friday, security forces and plain-clothes police had killed about 220 people in their efforts to clamp down on protesters.
Yitzhak Benhorin, in Washington contributed to this report
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