The toppling of Syrian President Bashar Assad's regime would serve a serious blow to Iran and would serve to further isolate the Islamic Republic, a top U.S. official said on Tuesday, adding that change in Syria was "inevitable."

President Barack Obama's National Security Advisor Tom Donilon, speaking a day after the United States announced new measures against Iran over its nuclear program, said that "end of the Assad regime would constitute Irans greatest setback in the region yet—a strategic blow that will further shift the balance of power in the region against Iran."

Open gallery view White House National Security Advisor Tom Donilon and U.S. President Barack Obama, November 18, 2011. Credit: Reuters

Referring to the new set of Iran sanctions, which mainly targeted the country's energy sector, Donilon said that Washington was "certainly not ruling out additional steps against Irans banking sector, including the Central Bank. As we do all this, we are not taking any options off the table. No one should doubt that."

"Even as Tehran continues to engage in dangerous and destabilizing behavior, Iran today is fundamentally weaker, more isolated, more vulnerable and badly discredited than ever," the top Obama aide said, saying that "tremendous pressure at home" was causing divisions between the Supreme Leader Ayatollah Ali Khamenei and President Mahmoud Ahmadinejad.

Speaking on the little backing Iran receives in the region, Donilon said that Iran was "basically down to just two principal remaining allies—the Assad clique in Syria and Hezbollah."

"And, like Iran, they too are fundamentally at odds with the democratic forces now sweeping the region. The Assad regime is thoroughly isolated and universally condemned. The Arab League, appalled by the regimes brutality, has shown remarkable leadership and taken the extraordinary step of suspending Syrias membership," he added.

Donilon also commented on Turkey's stance against the Assad regime, saying "Erdogan government—which spent a decade deepening ties with Syria—says it will no longer be fooled by Assads promises, and today, Prime Minister Erdogan has joined the international chorus calling on Assad to step down."

Leaders of Iran, concluded Donilon, "have taken a great nation and an ancient civilization and turned it into a pariah that is unable to integrate or engage with the world. That is a tragedy..... Weakened at home, diminished in the region, and isolated in the world—this is the dramatic shift in Irans fortunes that has occurred over the past three years. In this sense, we have succeeded in changing the dynamic that was at work when President Obama took office."

"If Tehran does not change course, the pressure will continue to grow. Working with allies and partners, we will continue to increase sanctions," the Obama aide said, adding: "With our Gulf Cooperation Council partners, we will continue to build a regional defense architecture that prevents Iran from threatening its neighbors."

"We will continue to deepen Irans isolation, regionally and globally. And, again—even as the door to diplomacy remains open—we will take no option off the table. For our focus and purpose are clear. Pressure is a means not an end, and our policy is firm. We are determined to prevent Iran from acquiring nuclear weapons," he added.

Asked at the end of his speech about the chances of success with regard to Russia and China's unwillingness to cooperate - Donilon said: "We actually had very good cooperation with Russians and the Chinese as we built this unified effort to force the choice on Iran."