By Matt Spetalnick
WASHINGTON (Reuters) - President Barack Obama's top national security aide said on Tuesday a U.S.-led drive to isolate Iran had slowed its nuclear program and that there was still "time, space and means" to persuade Tehran to abandon atomic weapons ambitions.
National security adviser Tom Donilon defended Obama's Iran policy in a wide-ranging speech following criticism by Republican presidential contenders that the administration had not done enough to thwart Tehran's nuclear advances.
His remarks may also serve as an appeal to Israel for more time to let Washington's strategy work. There has been growing speculation about an Israeli strike on Iran's nuclear sites since a U.N. nuclear watchdog accused Tehran this month of covert atomic weapons work.
"Iran today is fundamentally weaker, more isolated, more vulnerable and badly discredited than ever," Donilon said at the Brookings Institution think tank a day after the United States, Britain and Canada slapped new sanctions on Iran's energy and financial sectors.
He said that after Iran rejected the Obama's early diplomatic outreach and continued defying the international community, the United States had worked to ratchet up sanctions, strengthen military ties with Tehran's neighbors and increase it isolation.
"The effect of these sanctions has been clear," Donilon said. "Coupled with mistakes and difficulties in Iran, they have slowed Iran's nuclear efforts ... Not only is it harder for Iran to proceed, it is more expensive."
Despite those claims, Obama - like predecessors George W. Bush and Bill Clinton - has been unable to persuade Iran to halt its nuclear program or come clean on its developments.
DOUBTS ABOUT SANCTIONS
Analysts said they did not believe the latest punitive steps would be any more effective in dissuading Iran from pursuing its nuclear plans, which Washington and its allies say is a cover for seeking nuclear arms.
Though Iran has acknowledged some economic damage, it dismissed the new sanctions, saying they would only boost popular support for a nuclear program it insists is solely for peaceful purposes.
The range of unilateral steps planned by Western powers reflects the difficulty of persuading Russia and China not to veto further sanctions at the U.N. Security Council.
While the West has been reluctant to deal too harshly with one of the world's biggest oil producers because of the risk to world markets, Donilon said: "We are certainly not ruling out additional steps against Iran's banking sector, including the central bank."
He said Washington remained resolute. "Put simply, the Iranian regime has not yet fundamentally altered its behavior, but we have succeeded in slowing its nuclear program," he said.
"The international community has the time, space and means to affect the calculus of Iran's leaders, who must know that they cannot evade or avoid the choice we have laid before them," Donilon said.