British Prime Minister David Cameron called an emergency security meeting on Wednesday after a man was killed near an army barracks in London, and two men were shot and wounded by police.
Media reports said the dead man was a British soldier who was hacked to death by unidentified assailants. Some reports suggested his attackers tried to behead him while shouting "God is greatest" in Arabic.
"This is a sickening and barbaric attack," Home Secretary Theresa May said in a statement.
Britain's government convenes meetings of its emergency security committee only to deal with incidents that have implications for national security. A security source told Reuters on condition of anonymity that it was too soon to say whether the incident was terror related.
Security was tightened in the area immediately after the attack but details were sketchy. Helicopters hovered above the residential area and nearby roads were sealed off by the police.