The United States and its allies believe the window to stop Iran from building a bomb is quickly closing, pushing conflict with the Islamic republic to the top of the Obama administration's national security worries in the midst of an election year.

After years of diplomatic deadlock, Iran's nuclear program has advanced to the point where experts say work on a bomb could begin within a year. That progress has moved the once far-fetched possibility of a pre-emptive U.S. or Israeli strike on Iran's nuclear sites to the forefront of the urgent debate over how to prevent Tehran from joining the nuclear club.

The prospect of a military strike on Iran is still unlikely. The U.S. insists diplomacy and economic coercion are its main focus, and a military strike would be its last option to stop an Iranian bomb.

The United States has a "very good estimate" of when Iran could produce a weapon, President Barack Obama said this week. He said that while he believes the standoff with Iran over its nuclear program can still be resolved through diplomacy, the U.S. has done extensive planning on a range of options.

"We are prepared to exercise these options should they arise," Obama said during an interview with NBC. He said Israel has not made a decision about whether to launch its own strike.

Iran claims its nuclear program is not aimed at building a bomb, but it has refused to drop suspect elements of the program.

In November, the International Atomic Energy Agency issued a scathing assessment of the Iranian nuclear program, calling it disturbing and possibly dangerous. The IAEA, a United Nations body, said it had "serious concerns regarding possible military dimensions" of a program Iran claims is not intended to build a weapon.

Close U.S. ally Israel is driving much of the burst of international attention now focused on the likelihood of an Iranian bomb and what to do about it.

"When a country that refers to you as a `cancerous tumor' is inching, however slowly, toward a nuclear weapons capability, it's understandably difficult to relax and keep quiet," said Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace.

Israeli Prime Minister Benjamin Netanyahu frequently draws parallels between modern-day Iran and Nazi Germany on the eve of the Holocaust. Last week, Defense Minister Ehud Barak said there is a growing global understanding that military action may be necessary.

For Obama, the threat that the United States might use military force must ring true to Iranian leaders while not sounding alarmist to Americans or jittery oil markets. He has been very cautious, which is why his recent, blunter words are notable.

With the clock in mind, the Obama administration is moving much faster than expected to apply the heaviest economic penalties yet on Iran and the oil trade it relies on. This week came a surprise announcement of new sanctions on Iran's central bank, a key to the regime's oil profits.

Previous rounds of penalties have not changed Iran's course, but the U.S. and Europe, which just approved a first-ever oil embargo, argue that they finally have Iran's attention. The new oil-focused sanctions are intended to cut the revenue Iran's rulers can collect from the country's oil business without roiling oil markets.

While Obama has until late June to make a final decision on how to implement even stronger financial sanctions, a person advising the administration on the penalties said an announcement probably would come well ahead of that deadline. The adviser spoke on condition of anonymity because the White House plan is not final.

Among the factors pushing up a decision: the possibility of a unilateral Israeli strike and the desire to avoid disrupting oil markets in the summer, when gasoline prices are usually already higher.

With Republican presidential candidates questioning Obama's toughness on Iran, the White House also has a political interest in appearing to take a proactive approach to enforce the sanctions, rather than simply responding to a congressional deadline, the adviser said.

The threat of military action is also used to strengthen the diplomacy.

Countries like China, a major buyer of Iranian oil, don't like sanctions but go along because opposing them may increase the likelihood of military action that would spike prices for the oil they buy, Sadjadpour said.

White House national security spokesman Tommy Vietor would not comment on whether the timetable is being moved up. He rejected the idea that the administration is under the gun.

"We said all options are on the table. That is not bellicose and that is not new," Vietor said. "What we're trying to do is lead Iran to make a choice."

Israel's president tried to reach out to Iranians with a message of peace, appealing for them to loom beyond the rising tensions. "We were not born enemies and there is no need to live as enemies," Shimon Peres said Wednesday.

But Israel has less time to act than the U.S. if it chooses to mount a strike alone, U.S. and other officials said. Because Israel has less firepower, its leaders assess that a unilateral strike would be most effective before summer. After that, by Israeli estimates, Iran may have been able to move too much of its nuclear operation underground, beyond the range of Israeli missile and bomb attacks.

There is another reason that Israeli warnings are growing louder. Although Israel and the United States generally agree on the technical questions surrounding an Iranian bomb, they disagree about how much time that leaves for diplomacy or a last-ditch military strike.

Israeli officials who favor a strike do not want to wait for Iran to amass enough material to build a bomb, a debatable moment that could be as little as six months away. U.S. officials are concerned that the ability to make a bomb is not enough justification for a strike. They have argued there is 18 months or more of flexibility before Iran would pose an immediate nuclear threat.

Matthew Kroenig, a nuclear expert at the Council on Foreign Relations who recently spent a year advising the Pentagon on Iran options, agrees that the window for an effective strike by either country is closing.

"The game is over" when Iran amasses enough highly enriched uranium for a weapon, Kroenig said. "If you wait until they screw together a nuclear bomb, it's too late."

Administration officials are in discussions with several countries, including Japan, South Korea, China and India, to try to get commitments on how much they may be willing to reduce their imports from Iran. Iran exports about 3 percent of the world's oil and increasingly has focused on selling to customers in Asia as Western markets have dried up.

Talks are also under way with Turkey, South Africa, Sri Lanka, Pakistan and Malaysia, all main buyers of Iranian crude.

Any sanctions the U.S. ultimately levies would probably target companies in countries that purchase oil from Iran, not central banks, the person advising the administration said.