General Klaus Naumann, the outgoing chairman of Nato's military committee, said the need to maintain unity among the 19 members meant Nato had been unable to use surprise or sufficient force.

"This cost time, effort and potentially additional casualties and the net result being that the campaign is undoubtedly prolonged," he said.

"We need to find a way to reconsider the conditions of a coalition," he added.

He also criticised the military capabilities of several of Nato's European members and Canada.

The BBC's Ben Brown: A senior Nato general says the alliance made mistakes

He said Yugoslav President Slobodan Milosevic could still achieve his aim of the mass deportation of ethnic Albanians, but Nato would ultimately prevail.

Mr Naumann's comments followed strong criticism of Nato's strategy by the London-based strategic think-tank, the International Institute for Strategic Studies.

Nato 'wrong' to rule out ground troops

The IISS said Nato was negligent to initially rule out ground intervention and to limit its air strikes in the early weeks.

IISS director John Chipman attacked Nato's restrictions on targets and altitude, which had been imposed for fear of air crew casualties.

(Click here to see a map of last night's Nato strikes)

The criticism came as Nato announced it had carried out its most effective day of strikes against targets in Kosovo.

Mark Laity in Brussels: There are significant results against forces in Kosovo

Spokesman Jamie Shea also said the alliance had hit 40 fixed targets elsewhere in Yugoslavia.

"No part of the Yugoslav army was spared," he said.

In addition, Nato said one of its aircraft had shot down a Yugoslav warplane in an air battle.

A Pentagon spokesman said the Yugoslav MiG-29 was shot down over Yugoslavia by an F-16 fighter.

Bus denial

However, Nato denied that it had bombed a bus carrying civilians in Kosovo on Monday.

The BBC's Lisa Holland: "Sources suggest Nato was not involved"

Mr Shea said that a comprehensive review had found no evidence linking Nato with the incident, in which Serb sources said 20 people had died and 43 had been injured.

The alliance said although several Nato aircraft had been in the general area, film of the bus indicated that it had not been attacked from the air.

The wreckage of the bus contained a number of bullet holes.

A Nato source said there had been skirmishes in the area between the Kosovo Liberation Army and Yugoslav forces.

Mr Shea said Nato commanders made a thorough check of all gun cameras from planes operating in the area and interviewed all pilots.

Montenegrin radio reported that five other vehicles - three civilian and two police - had also been hit in the incident.

'Massive' ethnic cleansing

On the ground in Kosovo, Serbian forces are reported to have resumed ethnic cleansing on a "massive scale".

Mr Shea said the arrival in Macedonia on Monday of 11,600 refugees, mostly from a town in the north of the province, was the biggest one-day exodus for a month.

Diplomacy moves on

Russia's special Balkans envoy, Viktor Chernomyrdin, has continued his efforts to find a diplomatic solution to the conflict during a visit to the US.

He held talks with the United Nations Secretary-General Kofi Annan in New York.

Following their meeting, Mr Chernomyrdin said that there would be a Russian element to any international presence in Kosovo, but confirmed that it still has not been decided whether it should be a military presence or not.

The UN Secretary General intends to appoint two special envoys of his own who will work closely with Russia in their attempts to search for a solution.

The Group of Seven major industrial powers plus Russia - G8 - are due to meet in Bonn on Thursday to discuss solutions to the conflict, the German Foreign Ministry has announced.

In Bonn, sources close to the German Government said Russia had agreed to a G8 proposal for the "deployment of an international civil and security presence" in Kosovo.

Russia's Itar-Tass news agency quoted Deputy Foreign Minister Alexander Avdeyev as saying: "For the first time an agreement is reached on all the future international presence in Kosovo under the UN flag."

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