Airlines with direct flights to the US have been told to tighten screening of mobile phones and shoes in response to intelligence reports of increased threats from al Qaeda-affiliated militant groups, US officials said on Thursday.

The officials singled out smartphones including iPhones made by Apple Inc and Galaxy phones made by Samsung Electronics Co Ltd for extra security checks on U.S.-bound direct flights from Europe, the Middle East and Africa.

U.S. security officials said they fear bombmakers from the Yemen-based al Qaeda in the Arabian Peninsula (AQAP) have figured out how to turn the phones into explosive devices that can avoid detection.

They also are concerned that hard-to-detect bombs could be built into shoes, said the officials, who declined to be identified because of the sensitivity of the issue.

A U.S. official said that other electronic devices carried by passengers also are likely to receive more intense scrutiny.

Airlines or airport operators that fail to strengthen security could face bans on flights entering the United States, the officials said.

The U.S. Homeland Security Department announced on Wednesday plans to step up security checks, but they offered few details on how airlines and airports will implement them.

An official familiar with the matter said the United States believes that while it is possible there may be some additional delays at security checkpoints, at most major airports passengers will not be seriously inconvenienced.

The official said most passengers taking long-distance flights arrive well in advance of scheduled departures, leaving time for extra screening. But he said the United States could not rule out disruptions in countries where airport infrastructure and security procedures are less sophisticated.

U.S.-based airlines had little to say about the enhanced security. American Airlines spokesman Matt Miller said the Department of Homeland Security had been in contact with American on the issue, but declined to comment further.

Luke Punzenberger, a spokesman for United Airlines Inc, said: "We work closely with federal officials on security matters, but we are not able to discuss the details of those efforts."

U.S. security agencies fear bombmakers from AQAP and the Islamist Nusra Front, al Qaeda's affiliate in Syria, are collaborating on plots to attack U.S.- or Europe-bound planes with bombs concealed on foreign fighters carrying Western passports, the officials said.

AQAP has a track record of plotting such attacks. Its innovative bombmaker, Ibrahim al-Asiri, built an underwear bomb used in a failed 2009 effort to bring down a Detroit-bound airliner, and his devices were implicated in other plots.

There was no immediate indication U.S. intelligence had detected a specific plot or timeframe for any attack.

U.S. officials say the United States has acquired evidence that Nusra and AQAP operatives have tested new bomb designs in Syria, where Nusra is one of the main Islamist groups fighting to overthrow President Bashar al-Assad. – Reuters