The US and other countries say they are concerned by "credible allegations" of fraud in Guyana's election.
US Western Hemisphere affairs official Michael Kozak said no candidate should declare victory or be sworn in "while serious questions remain".
President David Granger was defending a narrow parliamentary majority against Irfaan Ali of the opposition PPP party.
Mr Granger declared victory, but observers said official results for a key region were not credible.
In their joint statement on Friday, US, British, Canadian and EU officials said they were worried about alleged electoral fraud influencing the results of the 2 March vote.
"We call on President Granger to avoid a transition of government which we believe would be unconstitutional as it would be based on a vote tabulation process that lacked credibility and transparency," the statement said.
International observers earlier expressed concerns, saying vote counting for Region Four - the most populous electoral district - was "interrupted and remains incomplete".
Opposition leaders say the elections commission altered the area's results to give Mr Granger's Partnership for National Unity and Alliance for Change (ANPU-AFC) coalition a victory over the opposition PPP.
They have denounced the results and say the PPP won the election.
The electoral commission has not yet declared an official winner.
What were the parties campaigning on?
The election campaign was dominated by how to use the windfall from oil finds which could put Guyana among the top 10 oil producers in the world.
Production in the offshore oil fields - estimated to contain at least 8bn barrels - started last year.
Analysts question whether the country will be able to use the new riches to spark much-needed development, avoiding the so-called "oil curse" of economic mismanagement and corruption seen in other oil rich nations with weak institutions.
The government said it expected up to $300m (£230m) in oil revenues this year, a huge figure for its 780,000 inhabitants.