"The U.S. Department of Defense continues to have personnel on the ground in Yemen to support the U.S. State Department and monitor the security situation," Little said in a statement.
The State Department Tuesday ordered non-emergency personnel at the U.S. Embassy in Sanaa be evacuated and warned against traveling to the country. The embassy had been closed last week and was expected to remain closed until at least Saturday.
"As we have said, we are concerned about a threat stream indicating the potential for terrorist attacks against U.S. persons or facilities overseas, especially emanating from the Arabian Peninsula," State Department Spokesperson Jen Psaki said in a statement. "As such, the department is taking appropriate steps to protect our employees, including local employees and visitors to our facilities."
The State Department also issued a travel warning to U.S. citizens urging them to delay travel to Yemen or to leave Yemen immediately.
The State Department last week also issued a worldwide travel advisory for U.S. citizens after it said it received credible information of a terrorist attack on an unknown target.
The BBC reported Yemeni security forces were on high alert amid warnings of an imminent attack by al-Qaida in Sanaa, as the United States and Britain were pulling out their diplomatic staffs and urging their citizens to leave the country.
BBC Arabic quoted a Yemeni security source as saying "extraordinary and unprecedented" security measures were in place and armored vehicles were stationed at the presidential palace and other government and foreign facilities in the capital city.
Dozens of al-Qaida militants were reportedly filtering into Sanaa in recent days, the BBC said.
In London, the Foreign Office said all British Embassy staff were temporarily withdrawn, The Guardian reported. The Foreign Office also advised against all travel to the country.
The Yemeni government already announced plans to increase security at foreign embassies, sea ports and airports, and other facilities of strategic importance, such as oil pipelines and power grids, The Guardian said.
Yemeni security officials said they were searching for 25 specific al-Qaida in the Arabian Peninsula operatives suspected of planning attacks.
AQAP was blamed for the aborted Christmas Day 2009 plot to bomb an airplane as it approached Detroit and for explosives-laden parcels intercepted in 2010 aboard cargo flights.
In its travel warning, the State Department warned U.S. citizens of the "high security threat level in Yemen due to terrorist activities and civil unrest. U.S. citizens currently in Yemen should depart."
"The security threat level in Yemen is extremely high," the warning read. "Terrorist organizations, including al-Qaida in the Arabian Peninsula, continue to be active throughout Yemen. The U.S. government remains highly concerned about possible attacks on U.S. citizens [whether visiting or residing in Yemen], and U.S. facilities, businesses, and perceived U.S. and Western interests."
U.S. citizens remaining in Yemen despite the travel warning were told to limit non-essential travel within the country and make their own contingency emergency plans.
In Washington Monday, White House spokesman Jay Carney said an alert that forced U.S. embassy closures across the Middle East and Africa may remain in place all month.
"Our current information suggests that al-Qaida and affiliated organizations continue to plan terrorist attacks both in the region and beyond," Carney said during a media briefing.
"And our information suggests that they may focus efforts to conduct attacks in the period between now and the end of August," he said.