U.S. President Barack Obama has defended U.S. policy on Iran, saying Tehran is facing “unprecedented, crippling sanctions” and that the Iranian government is now isolated.

Speaking at a White House news conference Tuesday, Mr. Obama stressed that U.S. and Israeli intelligence officials think the tensions with Iran can still be resolved diplomatically.

He also said world leaders will not permit Iran getting a nuclear weapon.

With respect to speculation that Israel may strike Iran's nuclear facilities, President Obama said Israel is a sovereign nation that has to make decisions about how to preserve its security. He said he is aware of the historical precedents that weigh on Israeli leaders when they think about potential threats to their country.

President Obama also said he does not expect a breakthrough at the first meeting with Iran when the international community resumes talks about its atomic program. Earlier Tuesday, the so-called P5 plus 1 – the five permanent members of the U.N. Security Council and Germany – offered to resume long-stalled negotiations with Iran.

Mr. Obama is facing criticism from Republican presidential candidates regarding his policies on Iran. In speeches to a pro-Israel group in Washington Tuesday, three of the leading Republican hopefuls called for a tougher approach on Iran – from placing warships along the Iranian coast to destroying Iran's nuclear facilities.

President Obama criticized the Republican candidates Tuesday, accusing them of resorting to “bluster and big talk” on the threat from Iran.

In his news conference, Mr. Obama also defended U.S. policy on Syria, where forces loyal to President Bashar al-Assad have been attacking civilian areas.

He called the situation in Syria heartbreaking and outrageous, and said Mr. Assad's actions have been inexcusable. But he said deploying U.S. military forces will not solve the problem, and that Mr. Assad will ultimately be forced from power.

Reporters also asked Mr. Obama about the burning of Qurans in Afghanistan by U.S. troops.

The president said the anger over the accidental burning of Qurans concerns him, but that Afghan leader Hamid Karzai knows the violence directed at American troops is unacceptable. Mr. Obama also said he is confident the U.S. will be able to transfer responsibility for security in Afghanistan to Afghan forces by the 2014 deadline, despite some “bumps in the road.”