President Barack Obama has warned of a credible terrorist threat against America after the overseas discovery of two US-bound packages containing explosives aboard cargo jets. Obama said, the packages, which according to the officials originated in Yemen, had been addressed to two Jewish centres in Chicago area. However, the White House said, the latest terrorist plot would have no impact on Obama's upcoming trip to India and other Asian nations. Obama told reporters in a hurriedly-convened White House press conference that the packages were identified in Dubai and Britain. Obama announced that there would be additional screening of some planes in Newark and Philadelphia. He said, more protective measures would be taken for as long as it takes.Reports say, the US-bound air parcel intercepted in Dubai, bore Al-Qaeda hallmarks including a powerful explosive and mobile phone detonator concealed in a computer printer. A Dubai police statement today said the bomb was a complex and professional device made of PETN and lead – a highly explosive combination that can cause great damage. The other explosive was found in Britain's East Midlands airport – and destined for Chicago synagogues.