Uganda's police have raised the country's "terror alert", as domestic and US intelligence agencies warned of an imminent attack similar to last month's assault on a shopping mall in neighbouring Kenya.

Security staff have been recalled from leave and thousands of extra police deployed across the capital, Kampala.

"Our intelligence and that of the Americans show an imminent terror attack is likely," police spokesman Patrick Onyango told Reuters news agency on Friday.

The Ugandan police appealed for calm and people's understanding as the police carried out more random searches and said the force's staff would not be allowed to take leave until further notice.

Al-Shabab threat

Uganda had already heightened security after the attack on the Westgate shopping mall in Nairobi, claimed by Somali armed group al-Shabab. About 70 people were killed during the siege of the mall.

Al-Shabab also carried out the 2010 bombings in Kampala, targeting crowds gathered for the occasion of the football World Cup final in reprisal for Uganda's participation in an African Union force that swept into Somalia to neutralise rebels and help end prolonged war and anarchy in the country.

The US Embassy in Kampala said on Tuesday it was assessing reports that a "Westgate-style attack may soon occur in Kampala".

It also cautioned its citizens to be vigilant about visiting crowded areas.

Al-Shabab has issued several threats to again strike Uganda, a staunch western ally, unless it withdraws from Somalia where its troops are fighting as part of a 17,000-strong AU peacekeeping force.