The Associated Press
In this photo taken late Thursday, May 15, 2014, British tourists queue to prepare to leave the international airport in Mombasa, Kenya. A number of British tourists left the country by charter flight Thursday following a new travel advisory from the British Foreign and Commonwealth Office (FCO) advising against all but essential travel to Mombasa island. (AP Photo)

NAIROBI, Kenya — Armed Marines patrol the U.S. Embassy grounds in Nairobi in bullet proof vests and helmets. Increasingly frequent emergency drills tell embassy staff: "Duck and cover, duck and cover."

The U.S. embassy on Friday sent out a new travel alert to American citizens warning of a continued terrorist threat in a country where the U.S. Embassy suffered a devastating attack in 1998.

An earlier warning this week said for the first time that the embassy itself is taking new steps to increase security "due to recent threat information regarding the international community in Kenya."

Britain's government also warned its citizens this week to avoid the coastal city of Mombasa and beach towns nearby, prompting a travel company to cut short the vacations of hundreds of British citizens and fly them home.

Security concerns have long been high in Kenya because of its proximity to Somalia and the al-Qaida-linked terrorist group that operates from there. The latest high-profile attack came in September, when four al-Shabab gunmen attacked an upscale mall in Nairobi, killing at least 67 people. The 1998 embassy bombing killed more than 200 people.

Since the mall attack, Kenya has suffered numerous smaller bombings in Nairobi and Mombasa. Authorities, with the help of the FBI, also discovered a huge car bomb that could have caused massive damage.

"We know from experience whether it's been in Yemen where embassies have been attacked or in Benghazi where our consulate and ambassador was attacked, anything that is a symbol of a foreign country is a potential target," said Scott Gration, the immediate past U.S. ambassador in Kenya.

Gration, a retired U.S. Air Force major general who runs a technology and investment consultancy in Nairobi, said embassies "are always a target, whether you have a warning out or not, they tend to be a magnet for people that have ideological intentions."

The U.S. Embassy says that more than 100 people have been killed in shootings, grenade attacks and small bombs in Kenya over the past 18 months.

TUI Travel, which owns the British tourism companies Thomson and First Choice, canceled all its flights to Mombasa until October because of the new security alert.

"As a precautionary measure, we have also taken the decision to repatriate all customers currently on holiday in Kenya back to the U.K.," on both Thursday and Friday, a company statement said.

A spokesman for Britain's government in the Foreign Office said the warning was only for the area immediately around Mombasa. The evacuation of tourists was a private company initiative, not a government one.

Gration said that many tourism companies have insurance policies for its tourists that don't allow travelers to be in high-risk locations. He said Kenya's coast is a beautiful and mostly safe location.

"My belief is that everywhere there are issues and we all need to be prudent in when we go and where we go," Gration said. "So I don't travel at night, avoid big crowds and lock my doors. Whether you are in Newark, New Jersey or Nairobi, Kenya, we can all fall victim to crime or terrorism."