No specific, credible threats toward U.S. over July 4th holiday - document

WASHINGTON, June 30 (Reuters) - There are no specific or credible security threats against the United States known to law enforcement heading into the Fourth of July weekend, according do a government document seen by Reuters and three sources briefed on the matter.

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The sources, who asked to speak on the condition of anonymity, said there would be increased vigilance and heightened awareness across law enforcement to keep Americans safe as they celebrate the nation's Independence Day.

See images of the Istanbul airport attack:

An internal briefing document shared with government officials and seen by Reuters called for the Department of Homeland Security to be on alert following recent attacks, such as the mass shooting in Orlando, Florida, on June 12, and calls from Islamic State supporters for attacks in the Western Hemisphere.

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The absence of known threats does not ensure safety. There were no threats known before the Orlando massacre, in which a gunman who pledged himself to Islamic State killed 49 people in the largest mass shooting in modern U.S. history.

The document said mass transit systems are particularly vulnerable to attack. It also said an attack over the U.S. Independence Day holiday would garner media attention that would further the Islamic State's propaganda efforts.

There was no mention in the document of the attack that killed 42 people at Istanbul's airport on Tuesday.