KABUL (Reuters) - Afghanistan accused the United Nations on Saturday of intervening in the formation of President Hamid Karzai's next cabinet, less than a week into his new term.

Since being re-elected in a controversial poll in which a fraud investigation rejected more than a million of his votes, Karzai has been under intense pressure from his Western backers to introduce swift anti-corruption reforms.

The Foreign Ministry said in a statement that comments made by the top U.N. envoy in Kabul at a news conference on Thursday had "exceeded international norms and his authority."

Kai Eide had said then that "warlords and power-brokers" should not decide the shape of the next government and warned Afghanistan it should not assume that its strategic importance guaranteed it continued international support.

The Foreign Ministry statement said instructions by "some political and diplomatic circles and propaganda agencies of certain foreign countries" had "violated respect for Afghanistan's national sovereignty."

On Friday British Prime Minister Gordon Brown said Karzai risked losing international support unless he acted decisively to fight corruption.

Eide had said Karzai's next government, which he plans to unveil in just under three weeks, "should be composed of competent, reform-oriented personalities that can implement a reform agenda."

Washington has long called for a stronger and more accountable Afghan government to fight a Taliban insurgency which is at its deadliest since the Islamists were forced from power in 2001.

U.S. President Barack Obama is considering whether to send an additional 40,000 U.S. troops to Afghanistan.

(Reporting by Golnar Motevalli; editing by Andrew Roche)