Even for a country that prides itself on its revolutionary credentials, Iran has been unusually bellicose in recent weeks, rejecting a nuclear deal it had earlier appeared to embrace and threatening to build new uranium-enrichment plants in defiance of international restrictions.

One reason, Iran specialists say, is that the embattled regime fears showing weakness in the face of persistent domestic political opposition and rising foreign pressure. Some even question whether supreme leader Ayatollah Ali Khamenei, the Shi'ite Muslim cleric chosen to lead the country 20 years ago, is still in charge.

Ayatollah Khamenei may instead now be subordinate to the Revolutionary Guards and other paramilitary forces that keep his government afloat.

Patrick Clawson, an Iran specialist at the Washington Institute for Near East Policy, said the country's decision-making abilities appear paralyzed. That may be why Iran has so far failed to follow through with a deal it accepted in principle in October to transfer nuclear materials to Russia and France for further processing.

The deal would have relieved pressure for new sanctions against Iran by reducing the likelihood that it could produce nuclear weapons in the near future. But the accord became the object of political infighting in Tehran as soon as it was revealed.

Last week, President Mahmoud Ahmadinejad declared that Iran would build 10 new enrichment plants and produce its own fuel for medical purposes, increasing the concentration of a uranium isotope from 3.5 percent - the current level of Iran's stockpile - to 20 percent. Weapons-grade uranium must be enriched to more than 80 percent of the explosive isotope known as U-235.

The Iranian president had earlier defended the idea of sending out more than 70 percent of Iran's stockpile as a victory for Iran, because it would have allowed the country to continue to make low-enriched uranium. Opponents of Mr. Ahmadinejad, including parliament Speaker Ali Larijani, warned, however, that outsiders could not be trusted to fulfill their part of the bargain.

"They are not all reading from the same script anymore," said Mehrzad Boroujerdi, an Iran specialist at Syracuse University. While the Islamic republic is notoriously fractious, Mr. Boroujerdi said he had not seen "this level of chaos in the higher levels of power in Iran in quite some time."

A senior U.S. official, who spoke on the condition of anonymity because he is not authorized to speak to the press, credited the Obama administration's policy of outreach to Iran with creating some of those fissures.

"The Obama strategy has generated a real debate in Iran over nuclear issues," he said.

Saeed Jalili, the nuclear negotiator who tentatively accepted the nuclear deal in October, "was the supreme leader's man and Ahmadinejad's direct emissary," the U.S. official said.

The debate that erupted afterward reflects "a crack in the core" of the regime, the official added.

The official said that the U.S. still hopes that Iran will compromise and that its recent behavior could be a bargaining tactic meant to show that Iran will not bow to foreign pressures.

Those pressures are mounting.

Late last month, the board of the International Atomic Energy Agency (IAEA) harshly criticized Iran for building a secret uranium-enrichment plant and failing to cooperate fully with the U.N. nuclear watchdog. Russia and China joined the censure and a new U.N. Security Council resolution against Iran is likely early next year.

Mr. Ahmadinejad responded with defiance, announcing that Iran would build 10 new plants to process uranium. On Friday, other Iranian officials said Iran needs 20 new plants and would give the IAEA only six months' warning before starting up such facilities.

Mr. Clawson and other Iran specialists downplayed the threats as "bombast," noting that Iran has yet to finish either of its two known enrichment facilities and lacks enough indigenous uranium to fuel additional plants.

While Iran is known for abrupt policy changes and may still agree to a deal, Karim Sadjadpour, an Iran specialist at the Carnegie Endowment for International Peace, said he doubted that any government led by Ayatollah Khamenei and Mr. Ahmadinejad - and backed by a generation of military men who came of age during the 1980-88 Iran-Iraq war - would risk accommodation with the United States, especially at a time of continuing domestic upheaval following the disputed June 12 presidential election.

"Enmity toward the United States is a fundamental pillar of the revolution," Mr. Sadjadpour said. "Their default position is always defiance."

He added that elder statesmen with experience in dealing with the West, such as former President Ali Akbar Hashemi Rafsanjani, have been sidelined and that "no one in this current group is capable of dealmaking."

While some specialists see Ayatollah Khamenei as increasingly beholden to security forces, Mr. Sadjadpour said he thinks the supreme leader is still in charge to the extent that anyone is. He noted that the ayatollah appoints the leaders of the Revolutionary Guards and paramilitary Basij and replaces them every few years to prevent them from building independent power bases. The ayatollah also endorsed Mr. Ahmadinejad and is close to Mr. Larijani, the parliament speaker.

However, the leader has lost support among many Iranian clerics who openly question his continued ability to rule. And while the regime has arrested hundreds of opponents, it appears to fear moving against the top opposition leaders, Mir Hossein Mousavi and Mehdi Karroubi, who ran against Mr. Ahmadinejad in June and are both former regime insiders. New protests are expected Monday on Iranian campuses to mark "National Student Day," previously a regime-backed event.

In a speech Nov. 25 to members of the Basij, Ayatollah Khamenei demonstrated the depth of his distrust of the West and his fears about growing internal divisions.

He said Iran's enemies tried and failed in the 1980s to overturn the Islamic republic by supporting Iraq in the Iran-Iraq war. Now, he said, the priority is "soft war; meaning a war using cultural tools, infiltration, lies, rumor-mongering."

"They use advanced tools that exist today, communications tools that did not exist 10, 15, 30 years ago," he said.

The ayatollah said Iran's enemies had exploited the outcome of the elections and appealed to Iranians to "stand together against those few people who are fundamentally opposed to the revolution, who are fundamentally opposed to the country's independence." At the same time, he said, Iranian media and politicians should not be so quick to condemn others and deepen divisions.

"Careful attention must be made to judgment; recklessness in passing judgment will bring with it injury and great peril," he said.