"Breaches and irregularities were observed in a certain number of constituencies, preventing a significant number of votes from being taken into account, and could therefore put into question the sincerity of certain results," the diplomats said in a joint statement yesterday, pointing to issues in eight out of 38 constituencies.
The country's electoral commission (CENI) should "identify these cases and refer to them in the document transmitting provisional results to the Supreme Court", added the observers who include UN envoy Said Djinnit, US and French ambassadors, as well as representatives from the European Union and west African bloc ECOWAS.
The opposition coalition claims the polls were marred by a string of irregularities, including ballot stuffing, voter intimidation and minors casting votes.
But in an interview with AFP, Guinea's President Alpha Conde dismissed the opposition's accusations.
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"I don't pay attention to all this," Conde yesterday said.
"Every party has its view, but it is their responsibility to send their views to the Supreme Court, which is the only jurisdiction with the authority to decide," he said.
"I am waiting for the outcome."
According to partial and provisional results released 10 days after the vote, Conde's ruling Rally of the Guinean People (RPG) party is in the lead.
They should have taken place in Guinea within six months of the swearing in of Conde in December 2010, after his election the previous month in the first ever democratic vote for a head of state in a nation ravaged by political, military and ethnic violence.
However, the vote was pushed back with opposing factions unable to agree on how it should be staged, leaving the role of parliament to be played by an unelected National Transitional Council.
The last parliamentary elections in Guinea took place in June 2002 during the dictatorship of General Lansana Conte, who died in December 2008 after 24 years in power.