A day after the United States imposed fresh sanctions on Iran over its nuclear program, President Obama’s national security adviser argued Tuesday that the administration’s pressure campaign had left Iran’s leaders economically strained, diplomatically isolated and rent by internal divisions, The New York Times reports.

The official, Thomas E. Donilon, predicted that the next blow to Iran would come with the fall of its major regional ally, President Bashar al-Assad of Syria.

Answering critics of the administration, he dismissed fears that the withdrawal of the last American troops from Iraq next month would open the door to rising Iranian influence. The Iraqi people, he said, hold deeply unfavorable impressions of Iran, even those who support political groups backed by Tehran.

“Iran has failed in its effort to shape Iraq into a client state in its own image,” said Mr. Donilon, adding that the United States would maintain a “long-term strategic partnership” with Iraq after the troops leave.