Local and federal police officials in Washington are bracing for potential security threats on Sept 18, when a right-wing rally is planned to support more than 570 people charged with crimes related to the attack on the Capitol, according to US media reports.

About 50 percent of US citizens said they are "extremely concerned" or "very concerned" about the threat to the country posed by extremist groups based outside the US, according to a poll by The Associated Press-NORC Center for Public Affairs Research.

However, about two-thirds of respondents said they are "extremely concerned" or "very concerned" about the threat of extremist groups based in the US.

The survey was conducted from Aug 12-16 as the two-decade war in Afghanistan ended with the Taliban recapturing Kabul, the capital. The international community has called for concerted efforts to prevent the country again becoming a hotbed and haven for terrorism.