The Foreign Secretary Robin Cook said yesterday that a blockade of the Adriatic coastal republic must be considered if the Alliance is to starve President Slobodan Milosevic's war machine of its fuel supply.
His views echo those of the US Secretary of State, Madeleine Albright, and have growing support among Alliance strategists.
Nato said the month-long air campaign had destroyed 70 per cent of the Serbian oil reserves.
Military planners are urging allied governments to authorise the blockade of Montenegro, Serbia's fellow Yugoslav republic, to prevent him replenishing his stock.
Several Allied governments are believed to be extremely worried about such an escalation and point out it may need a specific UN mandate.
Mr Cook said: "If we are really to deprive Milosevic of fuel, it cannot be right to let oil in through the ports."
In Washington, Ms Albright said: "We are taking all kinds of steps to limit the ability of outside powers to deliver oil ... There is a way to visit and search ships and we are looking at a variety of ways to tighten the screws on Milosevic economically."
Nato spokesman Jamie Shea said: "We are looking at what the options are to ... screw the tap down still further ... but we have to be consistent with international law."
Montenegro's government is pro-Western, opposes Serbia's ethnic cleansing and has already taken in almost 60,000 Albanian refugees from Kosovo.
But the government in Podgorica is under pressure from a vociferous pro- Belgrade faction and is said to be the target of a destablisation campaign by Milosevic. Hardship caused to the civilian population there by allied action could strengthen the Milosevic faction.
Alliance strategists also feel their plans are being seriously undermined by lack of low-level attacks, say senior defence sources and that hampers efforts to combat Serbian ethnic cleansing in Kosovo, one of the main aims of the strikes.
Although Nato has total air superiority at high altitude, the lower levels are fraught with peril.
Milosevic has largely saved his integrated air defence system from action and that could be deadly for allied aircraft at low levels. Apache helicopter- gunships could be highly effective against armour in Kosovo, but they would also be vulnerable flying at low altitude.
Military strategists say allied aircraft are having to carry out the attacks from a much higher level than they may wish, which is one reason for "accidents" such as the bombing of a passenger train near Leskovac in southern Serbia and Nato's bombing of a refugee column near Djakovica.
Yesterday, 400 Muslim villagers fled over the border to Montenegro from Serbia saying Serb soldiers ordered them to leave their homes. Military observers say this is part of the Serb process to establish a new front line.Reuse content