By Alberto Dabo

BISSAU, July 4 (Reuters) - Washington challenged Guinea-Bissau's government over its sacking of a police chief who won praise for tackling Latin American cocaine traffickers, according to a diplomatic note seen by Reuters.

With its lawless, island-dotted Atlantic shoreline, the tiny West African state has become a major hub for smuggled Colombian cocaine on its way to lucrative markets in Europe and elsewhere, prompting calls for international action to stem the trade.

The U.S. note was dated June 13, four days after President Joao Bernardo Vieira's new government sacked Judicial Police Chief Orlando Antonio da Silva, who had won international praise for fighting the drugs trade, and his investigations director.

"The United States is profoundly concerned by the sacking of Orlando Antonio da Silva and his colleague," said the note, addressed to Vieira from the accredited U.S. embassy in neighbouring Senegal, a copy of which was handed to Reuters late on Tuesday by a member of the Bissau government.

"Under their leadership, the judicial police carried out important operations resulting in the seizure of large quantities of cocaine," said the note, written in Portuguese.

"Without having proof that they failed in their jobs, the Washington government has the impression that the fight against drugs trafficking has taken a step backwards since their dismissal," it added.

U.S. embassy officials in Senegal's capital Dakar, who have responsibility for Guinea-Bissau, were unavailable for comment due to the July 4 U.S. holiday.

Guinea-Bissau, a desperately poor former Portuguese colony of less than 1.5 million people, has seen repeated bouts of fighting and instability over the past four decades and most of its people survive by fishing or growing cashew nuts.

Large drug seizures including that of nearly 2.5 tonnes of cocaine in neighbouring Senegal in the past week have shown the spread of the trade around West Africa, which smugglers see as a soft entry point compared to well policed European shores.

As head of Bissau's judicial police, which has led efforts to fight the drugs trade, da Silva had made several high-profile drugs seizures in the past year.

But his attempts to tackle well-equipped Latin American organised crime syndicates were limited by a severe lack of resources, meaning his officers were sometimes unable to pursue suspects because they had no vehicles or fuel.

High-level corruption and incompetence have also undermined police operations. The interior minister has complained of official connivance in the drugs trade.

Da Silva proudly announced Guinea-Bissau's biggest ever cocaine seizure last September, netting 674 kg (1,486 lb), but a judge set free two Colombian suspects without explanation and the cocaine vanished from a state Treasury vault.

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