"Milosevic should know that if he decides to do something of that nature...he will be stopped. We have plans to stop him if he plans to take that direction," Solana told Reuters Television News in an interview.

Asked if Nato would use force to prevent Belgrade sending troops into Montenegro to oust the government of his outspoken critic, President Milo Djukanovic, Mr Solana said: "Yes. If they are preparing to go into Montenegro."

Earlier, UK Government officials accused Mr Milosevic of plotting a coup.

Correspondent Jacky Rowland: Montenegro may be next scene of Yugoslav military campaign

"We have evidence to show that he is preparing a coup against Montenegro," Defence Ministry official Edgar Buckley told the daily briefing on the Nato air strike against Yugoslavia.

Montenegro's president Milo Djukanovic has been critical of Mr Milosevic's treatment of the Kosovo Albanians and has refused to acknowledge the federal government's declaration of a state of war.

Re-shuffle creates tension

The coup accusations came a day after President Milosevic removed the army chief in Montenegro and replaced him with one of his own close allies, General Milorad Obradovic.

Seven other high-ranking officers were also removed in the reshuffle of senior military posts.

The UK's concerns over Montenegro were mirrored by Nato's headquarters in Brussels.

One senior Nato diplomat said of the coup rumours: "It's certainly a serious possibility given longstanding efforts by Milosevic to undermine President Djukanovic and to use the current crisis to move in a more docile government".

Correspondents say the move has raised tensions in Montenegro which have been growing since the air strikes started.

Last week the American Secretary of State Madeleine Albright called the Montenegrin president to warn him of rumours of a coup against him. Appeal for calm

Montenegro is the last element of the Yugoslav federation that is linked to Serbia.

Defence Correspondent Jonathan Marcus: "There are concerns about wider security."

The small coastal republic has so far managed to steer clear of the conflicts which have battered the region since 1991, and has cautiously fostered its ties with the West in the face of severe opposition from Belgrade.

Although President Djukanovic openly denounces the federal government as "illegal and illegitimate", he insists he has no plans to push for independence.

That is unsurprising when opinion polls in the country show little more than 40% of the population wants secession.

At last year's general election, the biggest single party was the fervently pro-Milosevic Socialist People's Party (SNP), headed by Momir Bulatovic, the federal prime minister.

It attracted 36% of the vote but was kept out of office by a coalition of centre-left parties, led by Mr Djukanovic's Democratic Party of Socialists.

Delicate political balance

Some compare the situation here to that of Croatia in 1991 and Bosnia in 1992 when ethnic and political tensions exploded into war.

The crisis has seen 32,000 refugees flood into Montenegro

"There is a serious and authentic danger that even our state may disappear and burn down in the violence," Montenegro President Milo Djukanovic said this week.

Nato has limited its air strikes in Montenegro to attacks on key air defence radars so far, but BBC Defence Correspondent Jonathan Marcus says Nato might begin to target other military bases there if an attempted coup looked likely.