Italy's Foreign Minister Franco Frattini also called for an urgent meeting of the UN Security Council, while the European Union denounced the timing of the violence coming on the eve of the holy month of Ramadan.
"I am appalled by the Syrian government's use of violence and brutality against its own people," Obama said in a statement.
"The reports out of Hama are horrifying and demonstrate the true character of the Syrian regime.
"In the days ahead, the United States will continue to increase our pressure on the Syrian regime, and work with others around the world to isolate the Assad government and stand with the Syrian people," he added.
Human rights groups said Syrian forces had killed nearly 140 people on Sunday as President Bashar al-Assad crushed dissent on the eve of the Muslim holy month of Ramadan. Of those killed, about 100 died when the army stormed the flashpoint city of Hama, they said.
EU foreign policy chief Catherine Ashton condemned the "totally unjustified assault" in a statement Sunday. "This attack and the continuing crackdown in other Syrian cities is even more unacceptable coming on the eve of the holy month of Ramadan," Ashton added. "The Syrian army and security forces have the duty to protect citizens, not to massacre them indiscriminately."
Italy's Frattini called for an urgent meeting of the UN Security Council.
"We request that the United Nations Security Council hold an urgent meeting and adopt a very firm position," he told public broadcaster RAI.
"We have asked for the ambassadors of European Union member states to meet in Damascus tomorrow," he added, denouncing the crackdown as "a horrible act of repression".
France also condemned the bloodshed.
"Syrian political, military and security leaders must know, now more than ever, that they will have to answer for their deeds," a French foreign ministry statement said.
"The continuing of the repression and atrocities against civilians is particularly unacceptable on the eve of the month of Ramadan and can only lead to more instability and violence across the country," it added.
German Foreign Minister Guido Westerwelle threatened new sanctions against Assad's regime.
"I am deeply shocked by what we are currently seeing in Syria," Westerwelle said in a statement, calling for an immediate halt to violence against peaceful demonstrators.
"If President Assad fails to change course, we and our partners in the EU will impose new sanctions," he said.
British Foreign Secretary William Hague said he was "appalled" at the crackdown.
"The attacks are all the more shocking on the eve of the Muslim holy month," he said.
Assad "is mistaken if he believes that oppression and military force will end the crisis in his country."
Turkey, which has already taken in thousands of Syrian refugees fleeing the crackdown, also urged a halt to the violence, in a foreign ministry statement carried by the Anatolia news agency.
"While Turkey was expecting (Syria) to work for a peaceful atmosphere during the holy month of Ramadan, it is deeply saddened and disappointed together with all the Islamic world by the current developments on the eve of holy month of Ramadan," it said.
Dutch Foreign Minister Uri Rosenthal "noted with horror" the violence in Hama.
"The use of violence against peaceful demonstrators is unjustifiable and unacceptable," the Dutch foreign ministry said in a statement.
Rosenthal called for tougher EU sanctions and a UN Security Council resolution condemning the crackdown.
China and Russia, which hold veto power on the UN Security Council, have consistently blocked a Western-drafted resolution condemning the crackdown in Syria.
European Parliament head Jerzy Buzek also condemned the attack "in the strongest possible terms."
"The use of heavy weaponry and the indifferent killing of innocent civilians cannot be justified under any circumstances," he said in a statement.
JomoSamaTuaran juga fikir apa yang orang lain fikir, sebauh kerajaan yang dipilih secara demokratik oleh rakyat, kini sedang di tekan oleh kuasa-kuasa barat untuk menyerahkan kuasa pada segerombolan pemberontak, ahli-ahli pembangkang di negara yg bergolak. Semata-mata kononnya untuk menunjukkan bahawa kuasa-kuasa besar itu pengamal demokrasi tulin di dunia ini. Apakah keadilannya untuk kerajaan Syria dalam hal ini, begitu juga pada kerajaan Libya, Mesir dan negara-negara yg senasib dengannya.???