"We strongly encourage all U.S. citizens to take appropriate precautions, as the security situation in Libya is volatile," said an "emergency message" posted on the website of the U.S. Embassy in Tripoli, the capital.
"Review your personal safety plans, remain aware of your surroundings, including local events, and monitor local news stations for updates," said the statement, found at tinyurl.com/usembassy-gov. "Maintain a high level of vigilance and take appropriate steps to enhance your personal security."
The statement said there was "no specific information pointing to specific, imminent threats against U.S. citizens, [but] the potential for violence and kidnappings targeting Westerners in Benghazi is significant."
The message differed from ominous warnings from Britain, Germany and the Netherlands that said there were indeed specific, imminent threats and urged their citizens to leave Benghazi immediately.
"We are aware of a specific, imminent threat to Westerners in Benghazi," Britain's Foreign Office advisory said.
"We advise against all travel to Benghazi and urge any British nationals who are there against our advice to leave immediately," it said.
The warning urged Britons still in Benghazi to travel only in groups and only during daylight hours.
It also said "a high threat from terrorism" existed elsewhere in Libya. "Attacks could be indiscriminate, including in places frequented by expatriates and foreign travelers," it said.
Germany and the Netherlands issued somewhat similar warnings.
The British and European warnings did not elaborate on the threats' precise nature.
But British newspaper The Independent cited defense sources Friday as linking Britain's warning to al-Qaida in the Islamic Maghreb, or AQIM, based in Mali.
A Libyan congressman told McClatchy Newspapers Libyan officials found a threatening message against Europeans outside a foreign-run company earlier in the week.
Ottawa's warning was similar to the others, urging Canadians to "leave immediately." It also described clashes pro- and anti-government clashes in the hilltop town of Bani Walid, one of two final strongholds, along with Sirte, halfway between Tripoli and Benghazi, in which forces loyal to the ousted Gaddafi regime remain strong.
U.S. Secretary of State Hillary Clinton told the Senate Armed Services Committee Wednesday AQIM's growing strength in the region threatened Libya's fledgling government and was creating instability elsewhere.
She said AQIM was being supported by weapons looted from Libyan leader Moammar Gadhafi's well-stocked armories after he was overthrown and killed in October 2011.
"This Pandora's box of weapons ... is the source of one of our biggest threats," she said.
"There's no doubt that the Algerian terrorists had weapons from Libya. There's no doubt that the Malian remnants of al-Qaida in the Islamic Maghreb have weapons from Libya," she said.
An AQIM offshoot has claimed responsibility for last week's deadly Islamist militant attack and hostage crisis at a remote Algerian gas plant near the Libyan border.
At least 37 foreigners, one Algerian and 29 kidnappers were killed in the four-day siege.
A senior Algerian official told The New York Times Tuesday several Egyptian members of the group that attacked the gas complex also attacked the U.S. mission in Benghazi Sept. 11, 2012.
That attack killed U.S. Ambassador Christopher Stevens and three other Americans.
Officials in Cairo had no comment on the Egyptian connection.
AQIM started out seeking to overthrow the Algerian government and institute an Islamic state. The group has since declared its intention to attack Algerian, Spanish, French and U.S. targets.
More than half a dozen major multinational oil and gas companies based in Europe have operations in Libya, including France's Total SA and Spain's Repsol SA.
None of the companies offered to disclose the number of Western expatriate employees they have in Libya, citing security reasons.
Repsol spokeswoman Kristian Rix told the Times Thursday her company remained in Libya and was "in coordination with the government, which is making every effort to maximize safety."
Libyan Deputy Interior Minister Abdullah Massoud told the BBC the Western reactions were unfounded.