WASHINGTON: The US State Department said Saturday it is "deeply troubled" by reports of violence and irregularities surrounding Uganda's presidential election, which saw Yoweri Museveni win a sixth term in office.

"The Ugandan people turned out to vote in multiparty national elections on January 14 despite an environment of intimidation and fear," department spokeswoman Morgan Ortagus said in a statement.

"We are deeply troubled by the many credible reports of security force violence during the pre-election period and election irregularities during the polls," she said.

Museveni extended his 35-year rule after an election which his main rival said was marred by fraud.

He was accused of crushing the opposition and media ahead of one of the most violent election campaigns in recent years, which saw the deaths of at least 54 people.

Ortagus called for "independent, credible, impartial and thorough investigations" into the reports of violence and fraud, and for those responsible to face justice.

She condemned attacks on political candidates, and said the State Department is "gravely concerned by harassment of and continued threats to civil society."

Museveni won with 58.6 percent of the vote, seeing off a stiff battle from former ragga singer Bobi Wine.

Wine was under heavy guard at his home on the outskirts of Kampala as the results were announced, with his party saying he was under "effective house arrest."

The government said it was merely providing him with security.