The State Department ordered all non-emergency U.S. diplomats to leave Lebanon Friday, citing security concerns as the White House and Congress debate military strikes on neighboring Syria.
"The potential in Lebanon for a spontaneous upsurge in violence remains," the State Department said in a written notice.
American civilians were also advised to leave the country in April after a similar travel warning was issued.
"U.S. citizens living and working in Lebanon should understand that they accept risks in remaining and should carefully consider those risks," the department said.
"The situation remains tense, and sporadic violence involving Hezbollah or other extremist or criminal organizations remains a possibility in many areas of the country," the statement added, referring to the Lebanon-based armed group allied with Syrian President Bashar al-Assad that has sent fighters into Syria.
The State Department issued a similar warning for Turkey, citing "threats against U.S. government facilities and personnel."
The statement instructed Americans in Turkey to be "alert to the potential for violence," and avoid demonstrations and large gatherings.
Armed Shia groups aligned with Iran are threatening to retaliate against American interests in Iraq if the U.S. intervenes in Syria.
The Wall Street Journal, citing anonymous U.S. officials, reported that the U.S. has intercepted an order from Iran to militants in Iraq to attack the U.S. embassy and other American interests in Baghdad.
Wathiq al-Batat, a cleric who leads the Mukhtar Army, an Iranian-backed militia, said members have selected hundreds of potential targets, which could include both official American sites and companies "associated with the Americans."
"There is a good level of coordination with Iran on this issue and I cannot reveal more. Batat said. "Each armed group will have duties to carry out."
Another Iranian-backed armed Shia group in Iraq known as Asaib Ahl al-Haq issued a statement saying that action against Syria "will set the region on fire. The interests of the Western countries will not be saved from this fire."
Officials in Iraq, which has already experienced unrest related to the Syrian civil war, say they are taking the warnings seriously.
The warnings comes as President Barack Obama continues lobbying efforts at the Group of 20 summit for military action to punish the Syrian government for its alleged use of chemical weapons against civilians. They had reportedly been under consideration since last week, when Obama first said he was contemplating military action against Assad's regime.
The U.S. government said the alleged chemical weapons attack killed more than 1,400 people near Damascus.
Al Jazeera and wire services