The United States embassy in Uganda issued an emergency warning to its nationals on Wednesday, saying a terrorist attack may take place soon in the capital Kampala.

"The US embassy has received information of possible terrorist threats to locations where Westerners, including US citizens, congregate in Kampala, and that an attack may take place soon," the embassy said.

"Out of an abundance of caution, the US mission has cancelled some non-essential events scheduled at local hotels in the coming days," it said, telling US citizens they "should expect increased security sweeps and delays when entering or exiting hotel areas."

The US embassy gave no further specific information on the threat, but the east African nation -- a trusted US ally -- has been attacked in the past by Somalia's Al-Qaeda-linked Shebab militants.

Uganda is a key contributor to the African Union's AMISOM force battling the Shebab inside Somalia.

In 2010 the Islamists carried out twin bombings in Kampala targeting a restaurant and a club where football fans were watching the World Cup final between the Netherlands and Spain, killing 76 people in the region's worst attacks in more than a decade.

The US embassy in Kampala last issued an emergency warning in September 2014. Ugandan and Western security sources later revealed they had intercepted a transfer of explosive suicide vests by suspected Shebab members and made 19 arrests.

US officials said at the time that the militants were determined to avenge the killing of their leader Ahmed Abdi Godane in a US airstrike carried out earlier that month.

The Islamists were also behind the September 2013 attack on the Westgate shopping mall in neighbouring Kenya's capital Nairobi which left at least 67 dead, and recently issued a call for fresh attacks against such locations.

The US has continued to strike the militants from the air, and last week announced it had also killed a senior Shebab figure, Adan Garar, who was linked to the planning of the Westgate mall attack.

The Shebab emerged from the Islamic Courts Union that controlled Mogadishu in 2006 before being pushed out by Ethiopian forces.

The group, however, still controls vast rural areas from where they launch regular attacks against AU troops and the country's internationally-backed government.