Federal Officials Boosting Security Efforts Inside US Homeland Officials said the latest measures are "out of an abundance of caution."

WASHINGTON, Aug. 3, 2013 -- Federal authorities are boosting security in the United States after intelligence agencies detected a credible threat to Western interests overseas and the government began closing diplomatic posts in some Muslim countries, according to homeland security officials.

The Department of Homeland Security is increasing security measures at airports, train stations and other transportation hubs, and expanding scrutiny of visitors coming into the United States, two officials told ABC News.

The FBI, meanwhile, is "working sources" and taking other "logical steps" to monitor any potential threat, an FBI official said.

Read: U.S. Shuts Embassies, Consulates in Muslim Countries

The officials said the latest measures are being taken "out of an abundance of caution," and the recent intelligence contains "no nexus" to the U.S. homeland. However, there is an "air of mystery" and "uncertainty of exactly what the target is," one official said.

"As always, our security posture, which at all times includes a number of measures both seen and unseen, will continue to respond appropriately to protect the American people from an ever-evolving threat picture," a DHS official said today in a statement to ABC News.

On Friday, DHS and the FBI sent a joint intelligence bulletin to local and state law enforcement agencies across the country, outlining the recent intelligence and urging authorities to remain vigilant.

Unlike many such bulletins sent to law enforcement agencies, the bulletin issued Friday was classified, reflecting the sensitivity and seriousness of the situation.

Also on Friday, the State Department issued a global alert to American citizens and announced that it would be closing more than 20 embassies and consulates.

"The Department of State alerts U.S. citizens to the continued potential for terrorist attacks, particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula," said the alert. "Current information suggests that [Al Qaeda] and affiliated organizations continue to plan terrorist attacks both in the region and beyond."

Read: Citing Al Qaeda Threat, U.S. Issues Global Travel Alert

The recent threat information is believed to be tied to Al Qaeda in the Arabian Peninsula, the Yemen-based group that sent the "underwear bomber" to blow up a plane over Detroit on Christmas Day 2009.

One U.S. official said the latest embassy closings in Yemen and elsewhere are themselves taking place out of an abundance of caution "because of what the government doesn't know."

Many European countries have said they also will be temporarily closing embassies in Yemen.