London ? Prime Minister Gordon Brown, Washington’s closest ally in Afghanistan, toughened his tone Friday with this harsh message for the Afghan leadership: Clean up your act — for real this time — or risk a cutoff of support.

In what 10 Downing Street billed as a major speech, Brown reflected public outrage over troop casualties by threatening to pull back support — and perhaps even additional troops — unless Afghan President Hamid Karzai cracked down on corruption. It was his first challenge since the Afghan leader was declared the winner of an election deeply marred by charges of fraud and ballot-rigging.

“I am not prepared to put the lives of British men and women in harm’s way for a government that does not stand up against corruption,” he said.

Brown’s stark warning came as NATO allies in Brussels advised American officials on what policy President Barack Obama should embrace in Afghanistan, where the eight-year campaign against Taliban insurgents has stalled, with rising casualties for Western forces.

European political and military leaders are anxiously waiting for Obama to decide whether to increase troops levels — as sought by the top U.S. and NATO general in Afghanistan — or redefine the goals of the NATO-led mission.

Brown’s challenge to the Afghan government also reflects the pressure he faces as public support for the conflict wanes in light of deaths among the British forces, including the loss this week of seven more soldiers.

Still, he stated that the effort to defeat Taliban insurgents is vital to British security, leading some lawmakers to say his threat to Karzai could not be taken seriously because Britain had no viable option but to stay and fight, regardless of the Karzai government’s compromised position.

Conservative Party lawmaker Liam Fox, the party’s defense spokesman, said Brown could not really reduce Britain’s commitment if its own well-being is at stake.

He said it was not helpful for Britain’s position to be “confused by mixed messages or empty threats.”