WASHINGTON (AFP/Reuters) - Afghan President Hamid Karzai and US envoy Richard Holbrooke engaged in a testy exchange over recent elections, a source said, underscoring potentially cooler US ties if Karzai wins another term. Holbrooke, a veteran US diplomat who is the special envoy to the region, pressed the Afghan leader on last weeks elections amid allegations of widespread vote-rigging, an official with knowledge of the meeting told AFP. It was a difficult meeting and there were some sharp exchanges in it, the official said in Washington on condition of anonymity. Holbrooke met with all of the candidates in Afghanistans second-ever presidential election and shared a meal with Karzai on August 21, the day after the vote. The thrust of the meeting was to respect the electoral process, let it take its course and be patient and to respect the results, whatever they are, the US official said. Karzai met twice with Holbrooke, after the presidential election, including a private lunch in Kabul that turned tense when the US envoy raised the possibility of a run-off. After that confrontation, the two finished dessert and shook hands, officials said. The official said Holbrooke reiterated to each candidate the public US line that Washington was neutral in the race and steered clear of recommending any new vote while waiting for complete results. Karzai, who has led Afghanistan since the aftermath of the 2001 US-led military operation that ousted the extremist Taliban regime, enjoys a narrow lead as results trickle in. His main competitor, former foreign minister Abdullah Abdullah, has railed against what he says is vast state-engineered fraud but has urged his supporters to be patient and work through the electoral system. US tensions with Karzai, in meetings with Holbrooke and a visiting delegation of US senators, reflected both election-time stress and growing discord in American relations with the man who has been leading Afghanistan since the Taliban was overthrown in 2001. Endemic government corruption and his close ties with former militia leaders have eroded Karzais support, both with the Afghan people and with Washington policymakers. The Obama administration was particularly disturbed by Karzais last-minute alliance with Uzbek General Abdul Rashid Dostum, officials said. He (Karzai) has hurt himself in the eyes of a lot of people, a Western observer close to US deliberations explained of Dostums role in Karzais campaign. US officials say Dostum, who fought for Afghanistans Soviet-backed Communist government and later switched sides repeatedly during years of factional civil war, may be responsible for war crimes. Karzai justified the move to Washington, telling officials he believed Dostum, who enjoys the overwhelming backing of ethnic Uzbeks in the north of Afghanistan, delivered key votes that could put him over the top. Karzai would need more than 50 percent of the vote to avoid a run-off, but partial tallies so far show a close race with his leading challenger, Abdullah Abdullah. Karzai enjoyed a warm relationship with former US president George W Bush, with whom he often consulted by videoconference. Bushs ambassador to Kabul, Zalmay Khalizad, was a frequent dinner guest of Karzai. Karzais relationship is widely seen as cooler with President Barack Obamas administration.