The Obama administration declared Wednesday that Syria 's government is losing control of the country and said the world must act to prevent the development of a dangerous and violent power vacuum.

"The Assad regime is losing control of Syria ," a senior administration official said, citing a bombing that killed the country's defense minister and two other associates of President Bashar al-Assad.

U.S. Defense Secretary Leon Panetta, noting sustained fighting in Damascus, said the situation was "rapidly spinning out of control."

"It's obvious that what is happening in Syria represents a real escalation in the fighting," Mr. Panetta said during a news conference at the Defense Department with the British defense minister, Philip Hammond.

Meanwhile, officials in Washington unveiled new U.S. sanctions against Syrian government officials and businesses tied to the country's biological and chemical weapons program as the crisis deepened in Damascus on Wednesday.

Treasury officials designated 29 senior government officials, as well as five companies with links to nonconventional weapons programs, and a company controlled by a "corrupt crony" of the Syrian president.

Mr. Assad's government has repeatedly clashed with opposition forces, and human-rights monitors estimate that more than 17,000 people have been killed since the antiregime uprising in Syria broke out in March 2011.

"Today's actions reflect the unwavering commitment of the United States to pressure the Assad regime to end the carnage and relinquish power," said David Cohen, Treasury undersecretary for terrorism and financial intelligence.

Fighting in the Syrian capital has grown more intense this week as rebel soldiers have moved reinforcements into Damascus . The Syrian government has responded with using helicopter gunships to strike at rebel positions on the outskirts of the capital.

Underlying U.S. reaction to the developments was an effort to pressure Russia into supporting punitive international measures to force a political transition in Damascus .

Moscow and Washington differ on whether a new United Nations resolution should include the threat of punitive sanctions to force a political transition.

"The next step internationally is for the world to focus on what comes next, and that means working with international partners to push for a political transition," the senior administration official said. "The planning has started but it needs to continue and increase in pace because the key is we don't want a vacuum."

Mr. Panetta said it was critical for the international community bring "maximum pressure" on Mr. Assad to step down.

Mr. Hammond said the violence in Syria was getting closer to "the heart of the regime."

"I think what we're seeing is an opposition which is emboldened, clearly an opposition which has access increasingly to weaponry, probably some fragmentation around the edges of the regime as well," Mr. Hammond said.

The two leaders also repeated warnings to Mr. Assad to keep control of his chemical weapons stocks. In recent days, U.S. officials have said they believe the Syrian regime has begun to move some of its chemical weapons from their storage facilities.

"It's something that we've made very clear to them that they have a responsibility to safeguard their chemical sites and that we will hold them responsible should anything happen with regards to those sites," Mr. Panetta said.

The defense leaders also warned Iran not to disrupt shipping in the Strait of Hormuz , through which an estimated 20% of the world's publicly traded oil flows.

On Tuesday, the U.S. announced a 20-nation exercise scheduled for September to practice counter-mine operations. Mr. Panetta said the U.S. was prepared for "all contingencies" in the area and has invested in weapons systems to counter any Iranian attempt to shut down travel through the Strait.

International sanctions against Iran for its nuclear program are taking hold this summer, prompting concern about retaliation by Tehran .

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