ISLAM QALA, Afghanistan — Two years ago, foreign engineers built a new highway through the desert of western Afghanistan, past this ancient trading post and on to the outside world. Nearby, they strung a high-voltage power line and laid a fiber-optic cable, marked with red posts, that provides telephone and Internet access to the region.

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Scott Eells for The New York Times

The battle for young hearts and minds plays out in Herat, where children play soccer in front of an American-built school; behind it, a school built by the Iranians.

The modernization comes with a message. Every 5 to 10 miles, road signs offer quotations from the Koran. "Forgive us, God," declares one. "God is clear to everyone," says another. A graceful mosque rises roadside, with a green glass dome and Koranic inscriptions in blue tile. The style is unmistakably Iranian.

All of this is fruit of Iran's drive to become a bigger player in Afghanistan, as it exploits new opportunities to spread its influence and ideas farther across the Middle East.

The rise of Hezbollah, with Iran's support, has demonstrated the extent of Tehran's sway in Lebanon, and the American toppling of Saddam Hussein has allowed it to expand its influence in Iraq. Iran has been making inroads into Afghanistan, as well. During the tumultuous 1980s and '90s, Iran shipped money and arms to groups fighting first the Soviet occupation and later the Taliban government. But since the United States and its allies ousted the Taliban in 2001, Iran has taken advantage of the central government's weakness to pursue a more nuanced strategy: part reconstruction, part education and part propaganda.

Iran has distributed its largess, more than $200 million in all, mostly here in the west but also in the capital, Kabul. It has set up border posts against the heroin trade, and next year will begin work on new road and construction projects and a rail line linking the countries. In Kabul, its projects include a new medical center and a water testing laboratory.

Iran's ambassador, Muhammad Reza Bahrami, portrayed his government's activities as neighborly good works, with a certain self-interest. Iran, he said, is eager to avoid repeating the calamities of the last 20 years, when two million Afghan refugees streamed over the border.

"Our strategy in Afghanistan is based on security, stability and de veloping a strong central government," he said. "It not only benefits the Afghan people, it's in our national interest."

Still, there are indications of other motives. Iranian radio stations are broadcasting anti-American propaganda into Afghanistan. Moderate Shiite leaders in Afghanistan say Tehran is funneling money to conservative Shiite religious schools and former warlords with longstanding ties to Iranian intelligence agencies.

And as the dispute over Iran's nuclear program has escalated [leading the United Nations Security Council to impose sanctions on Iran on Dec. 23], Iranian intelligence activity has increased across Afghanistan, American and Afghan officials say. This has included not just surveillance and information collection but the recruitment of a network of pro-Iranian operatives who could attack American targets in Afghanistan. [On Dec. 20 in London, British officials charged the interpreter for NATO's commanding general in Afghanistan with passing secrets to Iran.]

Discerning Iranian motives is notoriously difficult. Government factions often have competing agendas. Even so, the question of Iran's intentions in Afghanistan has come under a microscope in recent weeks amid debate in Washington over whether the United States should begin dealing with Tehran as part of a possible solution in Iraq. Some American officials have suggested that Iran's seeming cooperation in Afghanistan may be something of a model for Iraq.

So far, even as it declines to talk with the Iranians about Iraq, the Bush administration has adopted a posture of uneasy detente over Afghanistan. American officials say that they are watching closely, and no evidence has emerged of recent arms shipments to Iranian proxies, as there have been in Iraq, or of other efforts to destabilize the country. Iran's Shiite leaders appear to be maintaining their historic opposition to the Sunni Taliban, who consider Shiites heretics. Iran, they also say, is failing to gain popular support among Afghans, 80 percent of whom are Sunni Muslims.

Of far greater concern, according to American, European and Afghan officials, is Pakistan, America's ostensible ally against terrorism. They say the Pakistanis have allowed the Taliban to create a virtual ministate and staging base for suicide attacks just across Afghanistan's eastern border. Suicide attacks have quintupled, from 23 in 2005 to 115 this year, killing more than 200 Afghan civilians.

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The New York Times

A new highway built by Iran connects Herat to Iran and the outside world.

[It is too early to know if the Bush administration's position will be at all affected by the latest source of tension between Washington and Tehran - the American arrests of several Iranians in Baghdad on Dec. 20 and 21 on suspicion of conducting attacks on Iraqi security forces.]