The US has (warned) of a "specific, credible threat" ahead of the 10th (anniversary) of the 9/11 attacks, possibly in New York or Washington DC.
New York Mayor Michael Bloomberg said the (threat) was "uncorroborated", but security would be (boosted) at bridges, tunnels and on public (transport).
Police Commissioner Raymond Kelly said spot checks in the New York subway would be more (frequent) in coming days.
President Obama (called) on the US to "redouble" anti-terror efforts.
US TV network ABC News reports that three individuals - one a US citizen - entered the US in August aiming to (carry out) a terror attack. There was no (confirmation) about whether this information was linked to the threat (detailed) by US officials.
