Threats against the United States have doubled since the August bombings of two U.S.embassies in Africa  ALSO: Terror suspect with bin Laden ties held without bail RELATED VIDEO CNN's Pierre Thomas reports from Washington Windows Media 28K 56K

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• State Department precautions
• Tighter security
• Related sites and stories

WASHINGTON (CNN) -- Last week's bombing of Iraq has further increased the chance of a terrorist attack against Americans living and traveling abroad, U.S. officials tell CNN.

Serious threats against the United States and its citizens have doubled since the August 7 terrorist bombings of U.S. embassies in Kenya and Tanzania, according to a senior law enforcement source who declined to give specific numbers.

Law enforcement and intelligence sources say the missile attacks against Iraq, combined with U.S. air raids in August against alleged terrorist sites in Afghanistan and Sudan, have focused anti-American sentiment among suspected terrorist groups.

State Department precautions

As a precaution against retaliation, the State Department in the past week:

• Warned U.S. officials in the Persian Gulf of a possible attack in the next 30 days.

• Temporarily shut down most U.S. embassies in Africa.

• Issued a caution to U.S. citizens traveling abroad.

In addition, FBI rapid-deployment teams, which respond to large-scale terrorism strikes against Americans overseas, are at a high state of alert.

"Clearly, at a time like right now, when we anticipate the possibility of an act against the United States, (security) efforts will be stepped up," said Harry "Skip" Brandon, a former senior FBI intelligence official.

Tighter security

Security has been increased at airports

FBI sources say they have yet to receive any evidence of a specific attack. Still, the bureau has made a number of precautionary moves including:

• Mobilizing its new $20 million communications hub.

• Advising local police departments to closely monitor water supplies, telephone switching centers and electrical systems.

Other government agencies took precautions, too. Last week, security was stepped up at airports and thousands of federal buildings.

The General Services Administration, which oversees federal buildings, said security had been raised to a maximum state of alert from a heightened security alert, meaning more police presence and intensive screening.

Despite the infrequency of attacks on U.S. soil, threats of terrorism are routine. In fact, U.S. embassies receive thousands per year. But given the current climate, U.S. authorities are watching for any hint of terrorist planning or activity.