A serious kidnapping threat to Westerners in Baghdad has forced American diplomats to drastically curtail their movements ahead of the complete withdrawal of U.S. military forces from Iraq by the end of the month.

The growing threat illuminates concerns about the capacity of a stepped-up U.S. civilian effort to operate in Iraq once the military has gone, and in particular the risk that neighboring Iran will attempt to undermine American influence by using allied militias to abduct Western civilians.

The first warning of a “possible increased kidnapping threat” was posted on the Web site of the U.S. Embassy in Baghdad on Nov. 22. That was upgraded to a “significant threat of kidnapping” on Nov. 28 and then to a “severe threat of kidnapping” on Saturday.

For the first time, the advisories addressed to U.S. citizens specify that the threat applies to the Green Zone, also known as the International Zone, the heavily fortified enclave in central Baghdad where the U.S. Embassy is located and where most Iraqi government officials live.

Because of the threats, “the U.S. Embassy has greatly enhanced the security posture for U.S. Government employees,” said the latest warning. “This enhanced security posture includes severely restricted movement within the IZ.”

Westerners in the zone are venturing out only for short journeys and with armed escorts, said an American resident who asked not to be identified because he fears for his safety. Most embassy employees are confined to the fortified embassy compound, he said. Rumors that kidnappings have occurred are flying. “Everyone is going hysterical,” he said.

U.S. Embassy spokesman Michael McClellan declined to comment further on the nature of the threats or their impact on U.S. diplomats’ ability to function. “We are definitely not going to discuss any security arrangements . . . as a matter of policy,” he said.

But the threat underscored the difficulty the United States will face in wielding influence in Iraq. With only 9,000 troops left in the country, the U.S. military is on track to complete the withdrawal on schedule by Dec. 31, said Col. Barry Johnson, a spokesman for the U.S. military.

Though Sunni extremist groups linked to what is now known as al-Qaeda in Iraq were responsible for most kidnappings of Westerners in the early years of the war, U.S. officials and security experts say the biggest threat in Baghdad now comes from Shiite militant groups affiliated with Iran, which could use kidnappings to promote Iranian influence in Iraq at the expense of the West.

“The increased tensions between the West and Iran are raising concerns that Iran may single out Westerners for kidnapping to pressure Western governments,” said John Drake, a risk consultant in Baghdad with the British security firm AKE. Al-Qaeda in Iraq is still active, however, especially in the mostly Sunni northern and western provinces, he said.

The United States has pledged that it will not abandon Iraq after the troops go home, and Vice President Biden visited Baghdad last week to reinforce the message. The State Department plans an unprecedented diplomatic footprint involving as many as 16,000 people, 80 percent of them private security contractors. They will fulfill many of the functions that were performed by the military, including running a small airport and training Iraqi security forces.

The embassy has not relied on the U.S. military for security for many years, and it uses contractors to protect diplomats as they move around. But if the threat of kidnapping confines diplomats to the embassy, it will be harder for them to meet with Iraqis and carry out projects.

The efficacy of kidnapping as a tool to undermine political influence was demonstrated by the abductions of Americans and other Westerners by Shiite militants backed by Iran in Lebanon in the 1980s. Western journalists, aid workers, teachers and business executives fled the country as the kidnappings surged, leaving in tatters to this day a U.S. effort to turn Lebanon into a U.S. ally.

Hezbollah, the Shiite movement that has been designated a terrorist organization by the United States in part because of its links to those who carried out the kidnappings, controls the Lebanese government.

The last abduction of Westerners in Iraq was carried out in 2007 by the Asaib Ahl al-Haq group, which the U.S. military says is trained and funded by Iran. The only hostage who survived said after his release in late 2009 that the five Britons were held in Iran for at least some of their captivity.

The safety of the Green Zone was called into question last week after a suicide bombing outside the Iraqi parliament, the most serious breach of security since a suicide vest bombing there in 2007. Iraqi politicians, including Prime Minister Nouri al-Maliki and parliament speaker Osama al-Nujaifi, have competed to assert that they were the target of this latest attack.

The U.S. military handed full control of Green Zone security to the Iraqi army in June 2010.