The concerns come as regime helicopters fired on rebel stronghold towns and raging violence killed more than 100 people on Monday.
It also follows the release of a UN report revealing how the Syrian army uses kids as “human shields”.
Government helicopter gunships strafed rebel positions in Al Heffa, as well as the opposition stronghold of Rastan in the central province of Homs.
An activist broke down in tears as she told how tanks were parked on the edge of Al Heffa, a town of 30,000 set in rugged countryside near Turkey.
“They have never come this close before,” Sem Nassar said. “There’s only one doctor working to treat the wounded in the town.” Most residents had fled, she said.
Such reports prompted Washington to voice concerns that Assad’s regime is planning to carry out new atrocities.
It comes after the massacre of 55 people last week in Al Kubeir and at least 108 near Houla on May 25-26.
“The United States joins Joint Special Envoy Kofi Annan in expressing deep alarm by reports from inside Syria that the regime may be organising another massacre,” US State Department spokeswoman Victoria Nuland said.
Nuland said the tactics showed the Assad regime, cracking down on the most severe threat to his family’s four-decade rule, was “increasingly desperate”.
“What government voluntarily uses helicopters and fires from them on their own civilians if they’re not desperate? What government depends on a bunch of thugs in trucks – irregulars – if they’re not desperate?” she asked.
But Nuland again ruled out US military intervention.
Russia has strongly opposed the prospect of force to remove Assad, whose family has been a key ally of Moscow since the Cold War.
“The concern has been that putting foreign military forces into this situation … will turn it into a proxy war,” Nuland said.
A spokesman for UN-Arab League envoy Kofi Annan said he was “gravely concerned by the latest reports of violence coming out of Syria and the escalation of fighting” by both sides.