By Carlo Muñoz - 10/18/13 08:51 PM EDT
One of the hardest hit areas has been the Damascus suburb of Mouadamiya.
"The regime’s deliberate prevention of the delivery of lifesaving humanitarian supplies to thousands of civilians is unconscionable," she added.
The Syrian blockade of international aid to Mouadamiya and elsewhere in the country comes as Washington has increased efforts to bring those needed supplies into the country.
So far, more than $500 million in humanitarian aid alone has been spent by Washington, to help deal with the thousands of war refugees streaming out of Syria, according to the Pentagon.
Psaki also chastised the regime's practice of allowing limited evacuations of civilians from rebel-held territories around Damascus "as an excuse to attack those residents who remain behind."
Syrian troops have spent months hammering anti-government forces in and around the capital city in an attempt to pry those areas from rebel control.
"Those who are responsible for atrocities in the Damascus suburbs and across Syria must be identified and held accountable," Psaki said Friday.
Assad's forces have unleashed a barrage of heavy weaponry, from warplanes to long-range artillery to force rebel units to abandon their positions.
American intelligence also showed Assad's troops used chemical weapons in multiple attacks against rebel forces in Damascus.
U.S. forces were poised to begin military strikes against Assad in retaliation for the use of chemical weapons.
While Congress debated whether to grant the White House authority to intervene in Syria, a Russian-proposed disarmament deal put those strikes on indefinite hold.
So far, Assad has complied with the terms of the U.S.-Russian disarmament deal, offering up a comprehensive assessment of his chemical stockpiles and allowing international inspectors into the country.
Secretary of State John Kerry praised the Assad regime for its compliance with the disarmament plan.
"I'm not going to vouch ... for what happens months down the road, but it's a good beginning, and we should welcome a good beginning," Kerry said in a joint press briefing with Russian Foreign Minister Sergei Lavrov in October.