There is no sign of Syria abiding by an international plan to bring fighting to an end hours before a UN deadline, the White House said on Monday.
Under the peace deal, Syrian troops must withdraw from protest centres by Tuesday. But US officials said the situation appeared to have worsened.
Monday proved to be one of the bloodiest days of the uprising, with activists reporting more than 100 dead.
Earlier violence spilled across Syria's borders with Turkey and Lebanon.
Turkey protested angrily, summoning Syria's envoy in Ankara after two incidents along their joint border left two dead and many injured, including Turkish citizens.
Separately a Lebanese cameraman was shot dead on Lebanon's northern border with Syria.
The surge in violence came on the eve of the deadline for troops to withdraw from Syrian towns and cities under the international peace plan brokered by UN and Arab League envoy Kofi Annan.
"We certainly have seen no signs yet of the Assad regime abiding by its commitments, which is obviously quite unfortunate," White House spokesman Jay Carney told reporters.
The US also expressed outrage over the violence along Syria's border with Turkey.
"We join the Turkish government in calling for the Syrian regime to immediately cease fire," state department spokeswoman Victoria Nuland said.
"Not only has the violence not abated, it has been worse in recent days," she said.
UN Secretary General Ban Ki-Moon issued a statement deploring the violence on Syria's borders.
He also made a final plea for the Syrian administration to halt all attacks on civilians, his spokesman is quoted by AFP news agency as saying.
Day of bloodshed
International condemnation poured in amid mounting pessimism over the prospects for Mr Annan's peace deal. Earlier Turkey's foreign minister described the April 10 deadline for troop withdrawal as "void".
After earlier agreeing to Mr Annan's plan, Damascus on Sunday called for written guarantees from rebel fighters to end attacks and a promise from foreign states not to fund them.
It said it did not want the rebels to exploit any troop withdrawal to reorganise and rearm themselves.
The rebel Free Syrian Army said although it backed the UN-Arab League truce, it refused to meet the government's new demands.
Meanwhile violence intensified across the country on Monday.
Activists reported more than 100 deaths - among them at least 30 civilians who died in bombardment in the central province of Hama.
Harrowing scenes were reported in the town of al-Latmana, where 17 women and eight children were said to have been crushed under the rubble of their homes in the second attack on the area in days.
In the first of the skirmishes on the Turkish border at least two people died and many were wounded after people crossing from Syria into Turkey at Kilis were shot at by the Syrian army, the governor of Kilis told the BBC.
In the second incident at least two Syrian refugees and a Turkish translator were wounded inside a refugee camp after being hit by gunfire, possibly from stray bullets from clashes between Syrian rebels and troops.
Turkey has seen a sharp rise in the number of refugees coming over the border in the past week, and now hosts some 24,000 Syrians, including hundreds of army defectors.
In a separate incident, a Lebanese cameraman for Lebanon's Al-Jadeed television channel was shot dead when a film crew came under a hail of bullets from troops on the Syrian side of the border.
A Western diplomat has told the BBC that the UN Security Council is expecting a letter from Kofi Annan on Tuesday regarding the Syrian government's implementation - or lack - of a military pullback by the 10 April deadline.
Mr Annan will also briefly visit Syrian refugees on the Turkish border on Tuesday on his way to Iran, where he is expected to try to convince the Iranians to lean on the Syrian government to stop the bloodshed.
China has also urged both sides in the conflict to honour the peace deal. Correspondents say that Russia, Syria's main ally, may now have to play a crucial role. Syrian Foreign Minister Walid Muallem is in Moscow for talks.
The UN says more than 9,000 people have been killed in the uprising against Mr Assad's rule which began more than a year ago.
The Syrian government says 2,000 security personnel have been killed in the uprising and blames the violence on "armed gangs" and "terrorists".