Obama administration officials insisted to a Senate panel today that sanctions against Iran are working although they also conceded that the target of the sanctions, Iran’s nuclear program, continues to grow and expand.

The testimony came as President Obama  vowed that he will tighten sanctions against Iran in the wake of the alleged Iranian plot to assassinate the Saudi ambassador in Washington.

“We’re going to continue… to mobilize the international community to make sure that Iran is further and further isolated and pays a price for this kind of behavior,” Obama said.

David Cohen, under secretary for terrorism and financial intelligence at the Treasury Department, told the Senate Banking Committee that the Treasury Department has worked for years to ensure that Iran’s financial system is isolated and impaired.

“We had seen a steady decline in access for these designated Iranian banks to the financial sector before CISADA,” Cohen told the committee referring to the Comprehensive Iran Sanctions, Accountability, and Divestment Act . “After CISADA, the line dropped straight down.”

Wendy Sherman, undersecretary of political affairs at the State Department, said the U.S. has also pressed China with some success to abide by the sanctions.

“We have seen reports that this has had an effect,” she said. “As far as we know Chinese companies have not finalized any new refinery construction projects since the enactment of CISADA.” She added, “They have been… much more lashed up with us, the EU, and Russia in this regard.”

Nevertheless, the panel said that Iran’s nuclear program continues to expand. “We have very strong P5+1 consensus that is very crucial for maintaining the most robust sanctions regime it has ever had,” Sherman said. “It has not deterred Iran’s nuclear program, but we believe it is making progress to do exactly that.”

Sherman called the assassination plot “a dangerous escalation of the Iranian government’s role in sponsorship of terrorism.”

Cohen added that it “is a dramatic reminder that the urgent and serious threat we face from Iran is not limited to their nuclear ambition.”

In return Sherman said, “We have talked to every capital in the world and we have encouraged them to enforce the sanctions that are on the books.” Additionally, she said administration is trying to “ratchet up” pressure and will “consider any option” to deal with Iran’s ambitions and terrorist activities.