WASHINGTON -- As President Obama faces pressure to back up his year-end ultimatum for diplomatic progress with Iran, the administration says that domestic unrest and signs of unexpected trouble in Tehran's nuclear program make its leaders particularly vulnerable to strong and immediate new sanctions.

The long-discussed sanctions would initiate the latest phase in a strategy to force Iran to comply with United Nations demands to halt production of nuclear fuel. It comes as the administration has completed a fresh review of Iran's nuclear progress.

In interviews, Mr. Obama's strategists said that while Iran's top political and military leaders remained determined to develop nuclear weapons, they were distracted by turmoil in the streets and political infighting, and that the drive to produce nuclear fuel appeared to have faltered in recent months.

The White House wants to focus the new sanctions on the Islamic Revolutionary Guards Corps, the military force believed to run the nuclear weapons effort. That force has also played a crucial role in the repression of antigovernment demonstrators since the disputed presidential election in June.

Although repeated rounds of sanctions over many years have not dissuaded Iran from pursuing nuclear technology, an administration official involved in the Iran policy said the hope was that the current troubles "give us a window to impose the first sanctions that may make the Iranians think the nuclear program isn't worth the price tag."