The alliance said yesterday that the issue was proving highly complex and had not, as expected, been discussed by Nato ambassadors yesterday. Instead it is still being debated by the alliance's Military Committee, before the ambassadors are asked to give their agreement.
Robin Cook yesterday told MPs that 25 per cent of the oil supplies could still get through to Yugoslavia in spite of the European Union-wide embargo on oil.
The Foreign Secretary said not all Russian tankers could be stopped, but he hoped the dialogue with Russia, following the "very positive" talks in Moscow with Strobe Talbot, the US Deputy Secretary of State, could cover the oil embargo.
Mr Cook said the embargo would be given legal force by the EU this week, with the support of Cyprus, which has 2,000 ships under its flag. The ban would have the voluntary backing of the Nato members, and the support of the Organisation of Islamic Countries was being sought.
There would be a "sharp reduction" in the oil reaching Yugoslavia "if it doesn't stop every last drop of it going through ... It does not rule out Russia itself deciding not to go through," Mr Cook said.
The precise terms of the rules are sensitive because they will determine whether Nato ships have the right to stop and search vessels from countries which have not agreed to the ban.
That could lead to conflict with Russia which has said it will ignore the embargo and continue to supply Yugoslavia in accordance with its international obligations.
On Tuesday, Nato's Supreme Allied Commander in Europe, General Wesley Clark, called for aggressive rules of engagement, arguing that "any visit and search regime has, of course, to have appropriate rules of engagement to be able to use the threat of force". He added that "if it is approved by the NAC [North Atlantic Council] the officers dealing with the merchant ships will give them appropriate instructions".
Yesterday Jamie Shae, Nato's chief spokesman, struck a more emollient tone, arguing that the plan's legal basis still had to be agreed and adding: "We have to define the zone of operation, the number of ships, calculate how the rules of engagement would work and how we would limit the impact on Montenegro."
A decision to go ahead with a blockade was taken last weekend at the Nato summit in Washington, following the EU's decision to impose a ban on supplying oil to Yugoslavia, including Montenegro - Serbia's main source of fuel.
Although Nato sources had predicted an early decision, one said: "This decision will have to be made in the national capitals. Everyone has different positions."Reuse content