SYDNEY, Australia — Fiji’s army defied the Pacific country’s police force yesterday by seizing thousands of rounds of ammunition, sparking fears that a coup may be imminent.

Australia was preparing warships to evacuate 7,000 of its citizens. Its foreign minister, Alexander Downer, said he was “very concerned” about the possibility of a coup, which would be the fourth in Fiji in the past 20 years.

Fijian troops seized a consignment of ammunition recently shipped in from South Korea from a wharf in the capital, Suva.

The seizure of the 7.5 tons of ammunition was in direct contravention of orders issued by the country’s Australian police chief, Andrew Hughes, who said it was too dangerous to release it until the political situation calmed down.

The head of the armed forces, Frank Bainimarama, has threatened to topple Prime Minister Qarase unless he drops a bill offering amnesty to the plotters of the last coup, in 2000.

On Tuesday, the government failed in an attempt to replace the outspoken military commander, who is visiting Fijian troops in the Middle East — a move that infuriated the maverick officer.

“I’ll be back to see that Qarase and his cronies step down,” Commander Bainimarama told the Fiji Sun.

But Mr. Qarase said there was “no question” of him or his Cabinet resigning, increasing the prospects of an explosive confrontation.

During the past two months, Australia’s relations with its South Pacific neighbors have plunged to a new low. In addition to the tension in Fiji, Australia is struggling to manage standoffs with the Solomon Islands, East Timor, and Papua New Guinea, amid accusations that it bullies its smaller, poorer neighbors.

Australia has taken a more robust approach to the Pacific region since the attacks of September 11, 2001, fearful that failing states could be exploited by terrorists or criminals.