American agents are poised to take covert action against the terrorist group Hezbollah.

The Central Intelligence Agency has been authorized to move against it as part of a secret plan by President Bush to help the Lebanese government halt the spread of Iranian influence.

Senators and congressmen have been briefed on the classified “nonlethal presidential finding” that allows the CIA to provide financial and logistical support to Prime Minister Siniora.

Mr. Bush signed the finding before Christmas, after discussions between his aides and Saudi Arabian officials. Details of its existence, known only to a small circle of White House officials, intelligence officials, and members of Congress, have been passed to the Daily Telegraph.

It authorized the CIA and other agencies to fund anti-Hezbollah groups in Lebanon and pay for activists supporting the Siniora government. The secrecy of the finding means that American involvement is officially deniable.

The Bush administration hopes that the Siniora government, severely weakened after its war with Israel last year, will become a bulwark against the growing power of the Shiite sect of Islam, championed by Iran and Syria.

Mr. Bush has launched a fresh drive, supported by the Sunni states of Saudi Arabia, Jordan, and Egypt as well as Israel, to stop Iranian hegemony in the Middle East emerging from the collapse of Iraq.

The president is increasingly alarmed at the threat from Iran, which has infiltrated the Iraqi government and is training Shiite insurgents and supplying them with roadside bombs.

A former American government official said: “Siniora’s under siege, and we are always looking for ways to help allies. Hezbollah is the A-team of terrorism and Iran and Syria have not let up in their support of the group.” The former Saudi Arabian ambassador to Washington, Prince Bandar bin-Sultan, is understood to have been closely involved in the decision to prop up the Siniora administration.

The Israeli government, which views Iran as its chief enemy, has also been supportive. “There’s a feeling in Jerusalem and Riyadh that the anti-Sunni tilt in the region has gone too far,” an intelligence source said. “By removing Saddam, we’ve shifted things in favor of the Shia and this is a counter-balancing exercise.”