WASHINGTON — The White House on Thursday condemned the "outrageous" mass killing of civilians in Syria and urged all countries to end support for President Bashar al-Assad's "brutal and illegitimate regime."

Activists said at least 55 people, including women and children, were killed in Wednesday's assault by pro-regime militiamen on Al-Kubeir, a small Sunni farming enclave surrounded by Alawite villages in the central province of Hama.

"The United States strongly condemns the outrageous targeted killings of civilians including women and children in Al-Kubeir in Hama province as reported by multiple credible sources," White House spokesman Jay Carney said in a statement.

"This, coupled with the Syrian regime's refusal to let UN observers into the area to verify these reports, is an affront to human dignity and justice."

In a likely reference to Russia, the key supporter of Assad, Carney called for the international community to come together to support the "legitimate aspirations" of the Syrian people.

"We call once more on all nations to abandon support for this brutal and illegitimate regime, and to join together to support a political transition in Syria -- one that upholds the promise of a future for which far too many have already died," Carney said.

President Barack Obama's administration has stepped up economic pressure on Syria but has repeatedly ruled out the use of force, amid weariness in the United States over the Afghanistan war and after conflicts in Iraq and Libya.

General Martin Dempsey, the top US military officer, acknowledged frustrations over Syria but declined to discuss any possibilities for US intervention.

Diplomatic and economic pressure on Syria "is moving at a pace that is slower than anyone would want. I mean, the continued massacres are just deplorable," Dempsey, the chairman of the Joint Chiefs of Staff, told reporters.

"The pressures that are being brought to bear are simply not having the effect, I think, that we intend. But I'm not prepared to advocate that we abandon that track at this point," Dempsey said.

UN leader Ban Ki-moon earlier told the 193-nation UN General Assembly that monitors for the world body were shot at while trying to reach Al-Kubeir.

Farhan Haq, a UN spokesman, later told AFP that none of the observers were wounded and that the team had to abandon until Friday efforts to get into the village.

"There is no justification for this regime's continued defiance of its obligations under the Annan Plan, and Assad's continued abdication of responsibility for these horrific acts has no credibility and only further underscores the illegitimate and immoral nature of his rule," Carney said.

Ban's predecessor Kofi Annan, the UN and Arab League envoy to Syria, called for stronger international action to back his peace plan, which includes demands for Assad to pull troops and guns out of cities and halt violence so that political talks can start.

But a cessation of hostilities that officially started on April 12 has now all but collapsed.