The US has issued a worldwide alert asking its citizens to maintain a high level of vigilance when travelling abroad as al-Qaeda supporters may target American facilities, personnel or nationals in the wake of killing of the terror network's emir Ayman al-Zawahiri.

Zawahiri, who played a key role in the 9/11 attacks and later formed the group's regional affiliate in the Indian subcontinent, was killed in a drone strike carried out by CIA on Saturday at a house in a posh locality in the Afghan capital Kabul where he was sheltering to reunite with his family.

"Following al-Zawahiri's death, supporters of al-Qaeda, or its affiliated terrorist organisations, may seek to attack US facilities, personnel, or citizens, the alert issued by the Department of State said.

As terrorist attacks often occur without warning, US citizens are strongly encouraged to maintain a high level of vigilance and practice good situational awareness when travelling abroad, it added.

Worldwide cautions provide US citizens with general information regarding terrorist activities, political violence, and criminal activity that occur abroad, as well as specific recommendations on how to prepare for possible contingencies.

The Department of State remains concerned about the continued threat of terrorist attacks, demonstrations, and other violent actions against US citizens and interests overseas, it said.

The Department of State believes there is a higher potential for anti-American violence given the death of Ayman al-Zawahiri on July 31, 2022, it warned.

Current information suggests that terrorist organisations continue to plan terrorist attacks against US interests in multiple regions across the globe. These attacks may employ a wide variety of tactics including suicide operations, assassinations, kidnappings, hijackings, and bombings, the advisory added.

It asked US citizens abroad to monitor the local news and maintain contact with the nearest US Embassy or Consulate.

It also asked travellers to enroll in the Smart Traveler Enrollment Program (STEP) to receive security messages and to make it easier to locate them in an emergency.

The department uses these security messages to convey information about terrorist threats, security incidents, planned demonstrations, natural disasters, etc.

It said that private US citizens should not travel to any country to participate in armed conflict.

US citizens are reminded that fighting on behalf of, or providing other forms of support to, designated terrorist organizations can constitute the provision of material support for terrorism, which is a serious crime that can result in penalties, including prison time and large fines, it added.