TEHRAN, Iran » Even as the U.S. and Iran pursue negotiations on Tehran’s nuclear program, they find themselves on the same side of a range of regional issues surrounding an insurgency raging across the Middle East.

While the two governments quietly continue to pursue their often conflicting interests, they are being drawn together by their mutual opposition to an international movement of young Sunni fighters, who with their pickup trucks and Kalashnikovs are raising the black flag of al-Qaida along sectarian fault lines in Syria, Lebanon, Iraq, Afghanistan and Yemen.

The U.S., reluctant to intervene in bloody, inconclusive conflicts, is seeing its regional influence decline, while Iraq, which cost the U.S. $1 trillion and more than 4,000 lives, is growing increasingly unstable.

At the same time, Shiite-dominated Iran, the magnetic pole for the Shiite minority in the region, has its own reasons to be nervous, with the ragtag army of Sunni militants threatening Syria and Iraq, both important allies, and the U.S. drawing down its troops in Afghanistan.

On Monday, Iran offered to join the U.S. in sending military aid to the Shiite government in Baghdad, which is embroiled in street-to-street fighting with radical Sunni militants in Anbar province, a Sunni stronghold. On Sunday, U.S. Secretary of State John Kerry said he could envision an Iranian role in the coming peace conference on Syria, even though the meeting is supposed to plan for a Syria after the resignation of President Bashar Assad, an important Iranian ally.

To some, the Iranian moves reflect the clever pragmatism of Iran’s new president, Hassan Rouhani, and his foreign minister, Mohammad Javad Zarif, aimed at building their country into a regional power. To others critical of the potential rapprochement, the moves are window dressing aimed at lulling the West into complacency while Tehran pursues nuclear weapons and supports its own jihadis throughout the region.

Yet, even Iranians outside the reformist camp see the shared interests as undeniable. "It is clear we are increasingly reaching common ground with the Americans," said one of them, Aziz Shahmohammadi, a former adviser to Iran’s Supreme National Security Council. "No country should have an eternal enemy, neither we nor the United States."

With Iran as an island of stability in a region plagued by violent protests, sectarian clashes and suicide bombers, there are not that many options left for Washington, experts here say.

"We face the same enemy, and the enemy of my enemy is my friend," said Mashallah Shamsolvaezin, a prominent Iranian reformist journalist who closely follows the Arab world. He recalled how Iranian intelligence operatives gave reliable information to American special forces troops battling Iran’s enemy, the Afghan Taliban, in 2001.

While the Obama administration acknowledges that Iran has the potential to be an influential player on regional issues from Afghanistan to Syria, senior officials have said they are keeping their focus tightly on the nuclear negotiations. Cooperation on any other issues, they said, hinged largely on coming to terms on Iran’s nuclear program.

The administration has concluded that Rouhani and Zarif have been empowered to negotiate on the nuclear program, but officials said it remained unclear whether their policy-making authority extended to regional issues like Syria. There, the Islamic Revolutionary Guards Corps holds vast influence through its Quds Force, and it is supplying weapons to Hezbollah in an effort to prop up Assad’s government.

The thaw in relations extends back almost a year, with the two countries making overtures long thought impossible, deeply angering Washington’s closest regional allies, Saudi Arabia and Israel.

As early as last spring, a series of secret talks in Oman and Geneva laid the groundwork for re-establishing relations, cut 34 years ago after Iranian students took U.S. diplomats hostage in revolutionary Tehran.

In September came the agreement — credited to President Vladimir V. Putin of Russia but fully backed and partly engineered by Iran — to remove Syria’s chemical weapons. Not long afterward, President Barack Obama and Rouhani held a historic phone conversation, and in late November the U.S. and other world powers struck a temporary nuclear agreement with Iran, the first in 10 years.

Iran has been presenting itself as the voice of reason, pointing at the extremely graphic videos of beheadings and other executions produced by some of the insurgent groups in Syria, while Rouhani wished a happy New Year to all Christians on his Twitter account.

"Now extremists are once again threatening our security, and as in 2001, both countries will cooperate with each other in Iraq, and potentially elsewhere too," Shamsolvaezin said. "This is the beginning of regional cooperation."

The thaw presents dangers to Obama and Rouhani, who will remain vulnerable to criticism from conservatives in both countries. Kerry’s invitation on Sunday for Iran to join "on the sidelines" of the Geneva conference was angrily rejected by Iranian hard-liners.

"The Americans are confessing Iran stands for peace and stability in this region," said Hamid Reza Tarraghi, a hard-line political analyst, with views close to those of Iran’s leaders. "But when they invite us for a conference on Syria we are ‘allowed’ to be present on the ‘sidelines.’ This is insulting."

Even Foreign Minister Zarif rebuffed Kerry, saying that "everybody must be unified in order to fight the terrorists," the official Islamic Republic News Agency reported.

But Tehran’s full participation in the conference would seem to present even deeper problems, in that the talks are aimed at planning for a Syria after Iran’s longtime ally, Assad, has stepped down.

Critics of U.S. policy say that the Obama administration is strengthening Iran at the expense of traditional allies, particularly Saudi Arabia and Israel. They say that Iran has not cut back on its support of its regional allies, like Hezbollah, the militant Shiite group in Lebanon, and Assad, and is deeply involved with Iraq’s Shiite government.

Moreover, they say, a final nuclear agreement with Iran, should it be reached, would relieve Iran of crippling economic sanctions, reviving its economy and giving it more resources to spread its influence in the region, while depriving the West of diplomatic leverage to restrain Iran.

Analysts in Iran say that Tehran is pursuing a clever strategy, using the U.S. to undermine its greatest regional rival, Saudi Arabia.

"Cooperating skillfully with Russia, Iran has managed to change the game both in Iraq and in Syria," said Hooshang Tale, a Tehran-based nationalist activist and a member of Parliament before the 1979 Islamic Revolution. "If we play our cards well, we will end up outsmarting both the U.S. and Saudi Arabia."

He and others note that Iran has managed to keep Assad in power and wields considerable influence over its neighbors, Iraq and Afghanistan. Rightly or wrongly, they view their regional enemy Saudi Arabia as being on the verge of collapse, saying in Friday Prayer speeches and in televised debates that the kingdom is ruled by old men who have lost their way.

"We are worried for Saudi Arabia, which seems weak and potentially unstable," said Shahmohammadi, who heads an institute that promotes dialogue between Sunni and Shiite Muslims. "Even we, as their competitor, see all the horrible consequences if things go wrong there."

On Tehran’s streets, where people tend to see much of the region as distant lands filled with mayhem and unrest, many Iranians welcome every step that brings Iran and the U.S. closer together.

"The U.S. stands for progress, for work, a future, new cars and a better life," said Mohammad Reza Barfi, an auto mechanic. "I’d rather have peace with the U.S. than with any regional country. What do they have to offer?"

© 2014 The New York Times Company