WASHINGTON (AFP) - US top military officer Admiral Mike Mullen on Wednesday warned Afghan President Hamid Karzai to crack down on rampant corruption, piling pressure on Kabul as Washington weighs a troop buildup.

"We are extremely concerned about the level of corruption and the legitimacy of this government," Mullen told reporters. "It's far too ... endemic."

Newly reelected Karzai "has got to take significant steps to eliminate corruption," the chairman of the US Joint Chiefs of Staff said.

"That means that you have to rid yourself of those who are corrupt, you have to actually arrest and prosecute them. You have to show those visible signs."

The admiral's comments underlined growing anxiety in President Barack Obama's administration over the legitimacy of Karzai's government after disputed elections tainted by allegations of widespread fraud.

With Obama facing a pivotal decision on whether to send US reinforcements to fight Islamist insurgents in Afghanistan, Mullen suggested a troop buildup made little sense if the Kabul government failed to earn credibility.

"If we don't get a level of legitimacy and governance, then all the troops in the world aren't going to make any difference," he said.

Obama has held off making a decision on a troop request from the commander in Afghanistan, General Stanley McChrystal, partly because of the political uncertainty surrounding the August election.

But Defense Secretary Robert Gates last month said Obama may have to make a decision on whether to send more troops even if the legitimacy of the Afghan government remains in doubt.

The US administration has made little secret of its concerns with Karzai's alleged graft and his pacts with unsavory warlords.

Obama now faces a difficult balancing act -- pushing Karzai to act without alienating the leader of a government allied with NATO against against Taliban insurgents.

The US president on Monday congratulated Karzai on his reelection but urged him to wipe out corruption. White House spokesman Robert Gibbs said Obama expected a "sustained effort" to improve governance.

Mullen said it was crucial that government at both the local and national level better serve the Afghan people.

"You have to have a governance, not just in Kabul... but also in provinces, in districts," Mullen said, adding that "the legitimacy really needs to be in the eyes of the Afghan people."

Mullen added: "He (Karzai) has to take ownership for his own country."

Having won office in an election plagued by charges of ballot-stuffing, Karzai on Tuesday pledged to get to work to eradicate corruption.

Karzai's brother, Ahmed Wali Karzai, head of the province of Kandahar, has been accused of having ties to Afghanistan's lucrative illegal opium trade.

The Afghan president enjoyed a warm relationship with Obama's predecessor, George W. Bush, after the United States and its allies overthrew the extremist Taliban regime and installed him in the wake of the September 11, 2001 attacks.

Obama was immediately cooler to the Afghan leader. Two key players in his administration, Vice President Joe Biden and regional envoy Richard Holbrooke, have had well-publicized bust-ups with Karzai.

Afghanistan is gripped by an increasingly violent Islamist insurgency despite the presence of more than 100,000 NATO-led troops, including 68,000 US forces.

Insurgent attacks have made 2009 the deadliest year for foreign troops -- as well as Afghan forces and civilians -- since the fall of the Taliban regime in 2001.