WASHINGTON, District of Columbia (AFP) - The United States (US) called on Thursday on Cambodia's political parties to negotiate their differences peacefully after election results handed a narrow victory to longtime ruler Hun Sen.

The opposition has alleged widespread fraud in the July 28 election and is contesting the tally announced by election authorities.

"We urge the leaders of Cambodia's political parties to return to the negotiating table and to work together for an outcome that serves the best interests of the Cambodian people," US State Department spokesman Jen Psaki said.

She called on all sides to "act in a peaceful manner" and "seek resolution of electoral disputes through dialogue rather than resorting to threats, intimidation, repression or violent civil unrest."

Ms Psaki repeated that the US was concerned by "reports of serious irregularities" in voting and supported a "full and transparent investigation."

Election results gave 3.2 million votes to Mr Hun Sen's Cambodian People's Party, with 2.9 million people casting ballots for the opposition.

Even with the lead for the ruling party, the results were closer than many outside observers expected as Mr Hun Sen has managed to stay in power for 28 years.

Shortly ahead of the vote, Cambodia issued a pardon that allowed the return of opposition leader Sam Rainsy from self-imposed exile.

Mr Hun Sen had faced threats from several US lawmakers to cut off assistance in the event of irregularities in the election.