Mr Cook remark's follow the Serb Government's banning of an interview between BBC World Affairs Editor John Simpson and Serbian opposition politician Vuk Obradovic.

Mr Obradovic, leader of the Social Democratic Party and a former Serbian general, has been a strong critic of President Milosevic and has called for him to step down before his regime tumbles.

Mr Cook described him as one of the "brightest stars" of Yugoslavia's military elite and said Mr Obradovic "remains well connected with senior military figures".

This led the foreign secretary to wonder how many other military figures shared their former colleague's views.

Vuk Draskovic: Could lead Serbs in opposition to President Milosevic

Although Mr Cook said all signs of dissent should be seen as significant, he added it was impossible to know how large the cracks in President Milosevic's government have become.

The foreign secretary's remarks echo those made earlier by International Development Secretary Clare Short.

She said Serbia's sacked deputy prime minister Vuk Draskovic could gain the support of enough Serbs to challenge President Milosevic.

Three government ministers have quit in Belgrade in protest at Mr Draskovic dismissal.

"Clearly it's a massive crack in the government and we all want a settlement as quickly as possible - this could be a very good sign," Ms Short said.

Clare Short: The resignations could be a very good sign

Ms Short's comments to the BBC came as the Russian Balkan envoy Yevgeny Primakov was reported as having concrete proposals to offer both Serbia and the West to end the conflict.

The Secretary General of the United Nations, Kofi Annan, is also meeting Russian leaders in Moscow on Thursday, as part of the effort to seek a ceasefire.

Clare Short: "The refugees want to go home"

The UK international development secretary stressed that multilateral co-ordination to provide aid and assistance for rebuilding Kosovo had already begun.

More than half a million refugees have fled the province since the Nato bombing began and witness reports of atrocities on the ground have continued.

Allied air attacks aimed at Serb forces and infrastructure on the ground have also hit Kosovo heavily.

During the daily Ministry of Defence briefing, a UK government spokesman said all Nato planes involved in strikes on Wednesday night returned safely after continuing raids on Serb communications and infrastructure.

British planes were heavily involved in the attacks, he said, with GR7 Harriers flying 11 missions, and Tornados and Sea Harriers flying six and four missions respectively.